This tells us (rather cryptically) where to look on the resource for the data to enter for the element:
ASWR+: Another Source Within the Resource + Others (see rule)
ASWR: Another Source Within the Resource
Any: Any sources
EPbRI: Evidence Presented By the Resource Itself (see 2.2.2.1)
Gazatteers: Gazatteers and other reference sources + (see rule)
IAiCoREE: Information Appearing only in the Content of the Resources Embodying the Expression
IAiCoREW: Information Appearing only in the Content of the Resources Embodying the Work
ISSN+: The ISSN register + Others (see rule)
OS: Other Sources of information (see 2.2.4)
OSAPiREE: Other Statements Appearing Prominently in the Resources Embodying the Expression
OSAPiREW: Other Statements Appearing Prominently in the Resources Embodying the Work
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)
REW: Resources embodying the work
RR: Resources and References
RS: Reference sources
SAoPSoIiREE: Statements Appearing on the Preferred Sources of Information (see 2.2.2) in Resources Embodying the Expression
SAoPSoIiREW: Statements Appearing on the Preferred Sources of Information (see 2.2.2) in Resources Embodying the Work
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 Edition + 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)