The BACK and BACK PGM paragraphs serve the same purpose.
The BACK paragraph is linked to a PAGE context, whereas the BACK PGM paragraph is linked to the PGM context.
These blocks are used as part of program calls that involve the *RETURN directive.
If the call was made in the INIT_PGM paragraph, the processing returned by the called program is routed to the BACK PGM paragraph.