proj to obj conversion can actually mean many things, but it's probably related to export of various 3D projects to 3D models saved in OBJ format. Generally speaking, projects are tied to the original software and this is the one you have to use for further export.
PRJ is much more common generic extension that PROJ and it might be old or uncommon tools that actually use .proj files, so as a while this conversion might be very specific or of limited use.
n case of 3D content it probably related to L3DT, export .proj files to optimized meshes saved in OBJ format.
Updated: August 14, 2023