The NCBI Handbook, Chapter 18 - The Reference Sequence (RefSeq) Database, Table 1:

AC_     Genomic     Complete genomic molecule, usually alternate assembly
NC_     Genomic     Complete genomic molecule, usually reference assembly
NG_     Genomic     Incomplete genomic region
NT_     Genomic     Contig or scaffold, clone-based or WGSa
NW_     Genomic     Contig or scaffold, primarily WGSa
NS_     Genomic     Environmental sequence
NZ_b    Genomic     Unfinished WGS
NM_     mRNA      
NR_     RNA      
XM_c    mRNA        Predicted model
XR_c    RNA         Predicted model
AP_     Protein     Annotated on AC_ alternate assembly
NP_     Protein     Associated with an NM_ or NC_ accession
YP_c    Protein      
XP_c    Protein     Predicted model, associated with an XM_ accession
ZP_c    Protein     Predicted model, annotated on NZ_ genomic records