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)