DITA maps are documents that collect and organize references to DITA topics to indicate the relationships among the topics. They can also serve as outlines or tables of contents for DITA deliverables and as build manifests for DITA projects.
Maps allow scalable reuse of content across multiple contexts. They can be used by information architects, authors, and publishers to plan, develop, and deliver content.
Default schemas that are used if one is not detected in the DITA map document are stored in the various folders inside [OXYGEN_DIR]/frameworks/dita/DITA-OT/dtd/ or [OXYGEN_DIR]/frameworks/dita/DITA-OT/schema/ (or if you are using DITA 2.x, inside [OXYGEN_DIR]/frameworks/dita/DITA-OT2.x/dtd/ or [OXYGEN_DIR]/frameworks/dita/DITA-OT2.x/schema/).
The default CSS files used for rendering DITA content in Author mode are stored in [OXYGEN_DIR]/frameworks/dita/css/.