Also remember that this is the 32nd century.
Luminescent ink.
Ink that changes depending on the light, like a mother-of-pearl effect.
Animated ink.
Ink that changes in response to any number of biological indicators, temperature, heart rate, adrenaline.
Ink that responds to bluetooth signals from a smart watch. Maybe it disappears so he can avoid getting a lecture from grandma.
As to content: Names, dates, locations of accomplishments, or perhaps more meaningful, of failures.
Bits of sage advice from mentors.
If the rank insignia of the service build upon each preceding rank, similar to the Army, USMC, and Air Force enlisted insignia, then get the rank insignia as a tat. And as the character progresses, add stripes/chevrons/etc to match. That could make for some drama if they ever get demoted, drummed out, or maybe just get the next chevron in anticipation of a promotion, only to be passed over. Now Captain Skippy is walking around with a Major's insignia on their arm when the promotion board just told him he's not Major material. Uh-oh.
Fun fact, my eyes start watering when I see a Visine commercial, so I'm writing this next part through tears.
Corneal tattoos.
