The following is my thoughts on aether, life force, the difference between the two, and how magic overall works in FFXIV. Your mileage may vary.
As I see it, everyone has a level of "excess aether" beyond that which is considered their life force. This is the personal well of aether that people draw on to perform their spells and the like. The size of this well differs from person to person, which is why you have situations like with the Cocobuki brothers in the THM guild - the one brother does not possess a large enough well to cast spells. Like other physical exertion, recovering from using this wellspring requires rest time or outside support (ethers and the like).
Also, since both this aether and your life force are effectively one and the same, you can use life force in place of your aether well. However, considering this is aether that is keeping you alive - it's not a very good idea. It is likely painful (Convert - you lose HP to do it), and likely fatal if used too much (CNJ quest line).
Using said aether is a tricky process. It is undefined, unshaped energy - and thus requires a manner of training and a focus in which to refine it into its intended purpose. THM use the gems in their staves as a focus, and ACN use the books and the formulae within them. Attempting to cast a spell of any significant power without such a focus would likely be difficult and the spell itself erratic and unpredictable without an intense level of mental fortitude.
The limits of these personal wellsprings are what led to the development of Black Magic. Rather than draw on one's own excess aether - you just take the aether from around you. Succor does this to a degree as well, which is why the clash between the two was so devastating to the world as a whole - aether was being directly ripped from the planet in great volumes in the War of the Magi. So the Elementals decided to cut this drain off at the source and just wiped it all clean.
Aether doesn't have to be forcefully taken, though. CNJ is proof enough of that - coaxing external aether from the elements around you. And, technically, the summons of both Scholars and Summoners is drawing on an outside aetheric source - elementals again and the remnant energy of Primals, respectively.
As for MNKs, I like to believe that their "chakra" also serve as a manner of focus - and "opening" them is basically learning how to use those key points in one's personal aetheric flow to focus and refine aether to perform great physical feats. Whether it's one's own flow of aether, or drawing from the world around them, I am uncertain. As much as I like the idea of another method of utilizing the ambient aether, I can very neatly see the process as just being a training method to utilize one's own aetheric resources.
To bring this back to the point at hand, though, I would be left wondering what source of aether would be involved in these tattoos? As I mentioned, if they're being used simply as a focus, then putting it on someone with a non-existent aetheric well would either a.) do nothing since they don't have the aether to cast it normally, or b.) draw on their life force instead, which would be horrendously dangerous. So, the tattoos would be only useful to those who could likely learn to cast the spell normally given the talent and training. As mentioned, possibly to avoid being disarmed without your usual focus or perhaps because they have issue tapping into this well with the usual methods.
However, if part of the creation of this tattoo involves infusing it with a bit of your own aether? Then we could have a situation of being able to provide an artificial wellspring. An outside source, like with the Elementals.
If this is the case, we have to wonder - how would such a thing work? Is that aether just there to cast the spell once and then it's done? Or would it somehow recover like one's natural aetheric wellspring? If it's the latter, then what would be the limit of this external aether source and how long would it take to recover?
As I see it, everyone has a level of "excess aether" beyond that which is considered their life force. This is the personal well of aether that people draw on to perform their spells and the like. The size of this well differs from person to person, which is why you have situations like with the Cocobuki brothers in the THM guild - the one brother does not possess a large enough well to cast spells. Like other physical exertion, recovering from using this wellspring requires rest time or outside support (ethers and the like).
Also, since both this aether and your life force are effectively one and the same, you can use life force in place of your aether well. However, considering this is aether that is keeping you alive - it's not a very good idea. It is likely painful (Convert - you lose HP to do it), and likely fatal if used too much (CNJ quest line).
Using said aether is a tricky process. It is undefined, unshaped energy - and thus requires a manner of training and a focus in which to refine it into its intended purpose. THM use the gems in their staves as a focus, and ACN use the books and the formulae within them. Attempting to cast a spell of any significant power without such a focus would likely be difficult and the spell itself erratic and unpredictable without an intense level of mental fortitude.
The limits of these personal wellsprings are what led to the development of Black Magic. Rather than draw on one's own excess aether - you just take the aether from around you. Succor does this to a degree as well, which is why the clash between the two was so devastating to the world as a whole - aether was being directly ripped from the planet in great volumes in the War of the Magi. So the Elementals decided to cut this drain off at the source and just wiped it all clean.
Aether doesn't have to be forcefully taken, though. CNJ is proof enough of that - coaxing external aether from the elements around you. And, technically, the summons of both Scholars and Summoners is drawing on an outside aetheric source - elementals again and the remnant energy of Primals, respectively.
As for MNKs, I like to believe that their "chakra" also serve as a manner of focus - and "opening" them is basically learning how to use those key points in one's personal aetheric flow to focus and refine aether to perform great physical feats. Whether it's one's own flow of aether, or drawing from the world around them, I am uncertain. As much as I like the idea of another method of utilizing the ambient aether, I can very neatly see the process as just being a training method to utilize one's own aetheric resources.
To bring this back to the point at hand, though, I would be left wondering what source of aether would be involved in these tattoos? As I mentioned, if they're being used simply as a focus, then putting it on someone with a non-existent aetheric well would either a.) do nothing since they don't have the aether to cast it normally, or b.) draw on their life force instead, which would be horrendously dangerous. So, the tattoos would be only useful to those who could likely learn to cast the spell normally given the talent and training. As mentioned, possibly to avoid being disarmed without your usual focus or perhaps because they have issue tapping into this well with the usual methods.
However, if part of the creation of this tattoo involves infusing it with a bit of your own aether? Then we could have a situation of being able to provide an artificial wellspring. An outside source, like with the Elementals.
If this is the case, we have to wonder - how would such a thing work? Is that aether just there to cast the spell once and then it's done? Or would it somehow recover like one's natural aetheric wellspring? If it's the latter, then what would be the limit of this external aether source and how long would it take to recover?
~