UDMF is a map format supported by ZDoom and Zandronum. It is a plain-text map format which does away with several of the binary lumps that the Doom and Hexen map formats have.
UDMF maps consist of three lumps: the header, TEXTMAP and ENDMAP. The header and ENDMAP are markers, while TEXTMAP contains the map data. ZDoom currently supports several other lumps:
- ZNODES: ZDoom GL/GL2 nodes. Normal nodes aren't recognized here.
- REJECT: Reject tables, same as Doom and Hexen.
- BEHAVIOR: Compiled ACS, same as Hexen.
- DIALOGUE: Strife conversation data.
Yes, that means that the nodes are optional, and that if nodes are omitted the port is expected to build their own nodes for the map.