Does SJP have to be CL (Control Language) program?
No, it could be written in any program language that supports the reading and writing of 5250 screens such RPG, COBOL, C or RDML (which is really RPG anyway).
If you have LANSA programs RDML is a good choice because it makes it very easy to call LANSA processes and functions and allows access to common inter-program communications mechanism such as the exchange list and data structures.