This tells us (rather cryptically) where to look on the resource for the data to enter for the element:
aswr: Any Source Within the Resource + Others (see rule)
aswr: Any Source Within the Resource
Any: Any sources
epbri_any: Evidence Presented By the Resource Itself + accompanying material or container + any source
Gazatteers: Gazatteers and other reference sources + (see rule)
ISSN+: The ISSN register + Others (see rule)
psoi: Preferred Source of Information + Others (see rule)
PSOIEI+: Preferred Source of Information from Earlier Iteration + Others (see rule)
PSOILI+: Preferred Source of Information from Later Issues or parts + Others (see rule)
RR: Resources and References
SSACPDoE+: Same Source as Corresponding Parallel Designation of Edtion + Others (see rule)
SSACPDoNRoE+: Same Source as the Corresponding Parallel Designation of a Named Revision of an Edtion + Others (see rule)
SSACPTPS+: Same Source As the Corresponding Title Proper of the Series + Others (see rule)
SSACPTPSS+: Same Source As the Corresponding Title Proper of the SubSeries + Others (see rule)
SSACPTPSS+: Same Source As the Corresponding Title Proper of the SubSeries + Others (see rule)
SSACTP+: Same Source As the Corresponding Title Proper + Others (see rule)
SSADN+: Same Source As the Distributor's Name + Others (see rule)
SSADoE+: Same Source As Designation of Edition + Others (see rule)
SSADoE: Same Source As Designation of Edition
SSADoNRoE+: Same Source as the Designation of a Named Revision of an Edtion + Others (see rule)
SSADoNRoE: Same Source as the Designation of a Named Revision of an Edtion
ssamn: Same Source As the Manufacturer's Name + Others (see rule)
SSAPoD+: Same Source As the Place of Distribution + Others (see rule)
SSAPoM+: Same Source As the Place of Manufacture + Others (see rule)
SSAPoPr+: Same Source As the Place of Production + Others (see rule)
SSAPoPubr+: Same Source As the Place of Publication + Others (see rule)
SSAPrN+: Same Source As the Producer's Name + Others (see rule)
SSAPubN+: Same Source As the Publisher's Name + Others (see rule)
ssatp: Same Source As the Title Proper + Others (see rule)
ssatp: Same Source As the Title Proper
SSATPS: Same Source As the Title Proper of the Series
SSATPSS: Same Source As the Title Proper of the SubSeries
SSATPSS: Same Source As the Title Proper of the SubSeries
stp: Series Title Page + Others (see rule)