The literal FA layouts use when a value does not apply, e.g., no disenrollment, a message with no reject code, or no paid claim.
FA record layouts tell you to enter NA in specific situations: Effective Disenrollment Date when the enrollee was not disenrolled; Reject Reason Code when a pharmacy message has no reject code; Pharmacy Message when a reject code has no message; and the Table 2IA paid-claim date, time, days-without-medication and patient-paid fields when the drug was never received. FA uses NA, not None, for these.
Sources: FA protocol, Table 1/2 Cols F, R, S, PDF pp. 11-13; Table 2IA Cols Q-T, Y, AA, PDF pp. 25-27; SNPCC protocol, Table 1IA Column IDs H-J, L-O, Q, R, T; Table 2IA Column IDs H, M, O, P, S-V (PDF pp. 15-21); CPE protocol, COA guidance, p. 12.