It is worth noting that TA can only be used for your first degree at a given level (AA, BA, MA). If I understand current policy correctly, it cannot be used for doctoral-level degree programs. That is policy, not law, but I don't see it changing anytime soon.
"For academic programs, Associate’s, Bachelor’s, or Master’s degree, TA may not be used for a lower or lateral degree program from the one the Soldier currently possesses. In addition to degree programs, TA is available to Soldiers to complete a high school diploma and to complete certificate programs. TA is not authorized for programs of study beyond a master's degree."
https://www.goarmyed.com/public/public_tuition_assistance_policies.aspx
GoArmyEd: Tuition Assistance (TA) Policies
The Tuition Assistance (TA) program provides financial assistance for voluntary off-duty education programs in support of a Soldier's professional and personal self-development goals. TA is available for courses that are offered in the classroom or by distance learning and is part of an approved academic degree or certificate program. The courses must be offered by schools that are registered in GoArmyEd, are accredited by accrediting...
As far as TA is concerned you can read through AR 621–5, but the only requirement I found was that you are a satisfactory participant. You register yourself for TA through the goarmyed website.
https://www.goarmyed.com/public/public_tuition_assistance_policies.aspx
GoArmyEd: Tuition Assistance (TA) Policies
The Tuition Assistance (TA) program provides financial assistance for voluntary off-duty education programs in support of a Soldier's professional and personal self-development goals. TA is available for courses that are offered in the classroom or by distance learning and is part of an approved academic degree or certificate program. The courses must be offered by schools that are registered in GoArmyEd, are accredited by accrediting...