The dtd collection
The dtd sub-collection within the system collection of the Content Repository contains the DITA 1.2 DTDs as supplied by OASIS, the group who develops and maintains the DITA standard.
If you are using DITA 1.3, this folder is empty and instead the CCMS uses the DITA 1.3 DTDs found in the org.oasis-open.dita.v1_3 plugin in the system/plugins folder.
The collection structure is an exact copy of the structure in which OASIS delivers the DTDs and in which they are integrated into the DITA Open Toolkit, for example.
IXIASOFT does not edit or modify these DTDs in any way and strongly recommends that you do not either. All of the additions made by IXIASOFT to these DTDs are made via standard DITA specialization rules and are found in separate DTDs specific to IXIASOFT. If you plan to create your own shells to use modified topic types, for example, it's important to be familiar with this collection structure because you will refer to files within these collections.
To learn more about these DTDs, refer to the OASIS DITA specification at oasis-open.org.