The full pathname must be used.
More information about UDIMs can be found here.
Automatically generates tiled and mipmapped TX textures. The TX texture will be linearized according to the colorspace parameter. There is also a button next to the filename to force the regeneration of TX files, and switching colorspace will regenerate the .tx textures if necessary and update the IPR.
Pepe model by Daniel M. Lara (Pepeland)