There are a number of categories of references included in the References Table. These categories are listed below with their tags. SWEREFs are tagged to allow easy access to the members of a category in case there is a need to update some or all members of the  category. 

nprNASA Procedural Requirements
npdNASA Policy Directives
hdbkNASA Handbook
llisLessons Learned - includes entries in the LLIS database as well as documents that contain Lessons Learned and are referenced in the SWEHB
tpTechnical papers, presentations and other non-NASA documents
paProcess Asset, typically from a Center. Some may be found in SPAN.
tngTraining courses
testSWEREFs reserved for testing