NCBI XREF Help Section
NCBI ASN.1 Modules
According to the ASN.1 standard, modules can contain public or private data types.  NCBI doesn't use private types.  The NCBI module files organize the ASN.1 data type into related segments of the NCBI ASN.1 data model.  For example, in the toolkit source code asn1 directory, the seq.asn file contains the ASN.1 data types that correspond to NCBI -Sequence ASN.1 data model.
The ASN.1 modules listed comprise the entire NCBI ASN.1 data model.  LARTS can translate any of the data types resident in these ASN.1 modules, provided the data is publicly available.   However, several of these ASN.1 modules support data only internally available to NCBI.
You can click on the data module to drill down to the NCBI ASN.1 data types.  For example, the NCBI-Seqset module maps to ASN.1 data types used to create a GenBank report as produced by the NCBI Entrez data retrieval service.
The NCBI ASN.1 Data Model conforms to a tree-like structure. For example, the Bioseq-set class data field defines the data type of Bioseq-set.  Most GenBank Bioseq-sets are nuc-prot (DNA with translating protein).  However, Bioseq-sets of class pop-set (population set) and  phy-set (phylogeny set) can in turn contain several Bioseq-sets of the nuc-prot variety. 
The ASN.1 modules can support other ASN.1 modules.  For example, the  NCBI-Sequence module contains ASN.1 data types used by the module NCBI-Seqset.
The Uses and Is Used By links provide a method of navigating the NCBI ASN.1 tree-like structure.
NCBI ASN.1 Modules and Contained Top Level NCBI Data Types
NCBI data modules and  the contained top level NCBI data types used in generating the most popular Entrez data reports are as follows.
GenBank Data --The  NCBI-Seqset module supports the Bioseq-set as a top-level data type.
EntrezGene - The NCBI-Entrezgene module supports the Entrezgene-Set type.
BlastOuput - The NCBI-BlastOutput module supports the  BlastOuput data type.  (See our
  NCBI Blast fixes/patches under the Products menu for ASN.1 output of this data type.)
SNPdb - The Module Docsum-3.0 supports the SNPdb data type ExchangeSet.