38 C.F.R. § 21.3025

Payment of subsistence allowance and special training allowance is prohibited to an otherwise eligible person—

(a) Who is on active duty and is pursuing a course of education which is being paid for by the Armed Forces (or by the Department of Health and Human Services in the case of the Public Health Service); or

(b) For a unit course or courses which are being paid for under 5 U.S.C. chapter 41.