Loop triangles are tessellated triangles create from polygons, for renderers or exporters that need to match Blender's polygon tesselation exactly. These are a read-only runtime cache. Tessfaces are a legacy data structure from before Blender supported n-gons, and were already mostly removed from the C code. Details on porting code to loop triangles is in the release notes. Differential Revision: https://developer.blender.org/D3539