The EID linkage rule variable determines which LINKEDMEMHEAD to select. By default the EID linkage rule is ENTITY:MEMRECNO.
Valid rules are:
ENTITY:MEMRECNO
ENTITY:MEMIDNUM
SAME_SRC:MEMRECNO
SAME_SRC:MEMIDNUM
DIFF_SRC:MEMRECNO
DIFF_SRC:MEMIDNUM
SRC_?:MEMRECNO
SRC_?:MEMIDNUM