Universal Dimension, or UDIM, is a UV mapping and texture system that utilizes tiles to offer more flexibility than the default UV grid map. One texture might be enough for the entire mesh of a simple 3D model, but if your model is extremely detailed, UDIM allows you to use multiple texture maps to address varying resolutions.
Currently, Sketchfab does not support the use of UDIMs when UV mapping your models.
However, Sketchfab does support regular UVs, including multiple UV sets on a single material channel.