YMT is the metadata cousin of YMAP. Where YMAPs say 'this object goes here', YMTs describe properties about objects - their bounds, their LOD behaviour, their variants. Glossary anchor.
What YMTs typically contain
Bounds and collision information. LOD distances. Texture variants. Audio tags. Spawn-region rules. Each YMT is small but informationally dense - the kind of file you don't think about until something doesn't load and the YMT was wrong.
Most modders touch YMTs only when introducing new props that need engine-side rules. Editing existing YMTs is fragile - a bad change can break loading.