GN get-next. If unqualified, this call retrieves the next segment in the hierarchical sequence of the database. If SSAs are specified, the call retrieves the next segment that satisfies qualifications specified by the SSAs.
GNP get-next-within-parent. This call is like the GN call but is restricted to the subtree of the current parent. (The parent is described in the PCB.)
I had a few day introduction to IMS in the 70's sometime. Never worked with it.