ITU used SDLC as a basis to develop LAPF for the Frame Relay environment, along with other equivalents:  LAPB for the X.25 protocol stack, LAPM for the V.42 protocol, and LAPD for the ISDN protocol stack.
ITU based LAPF on SDLC for Frame Relay, just like LAPB for X.25 protocols, LAPM for V.42 protocols, and LAPD for ISDN protocols.