You’re probably dealing with a large amount of triangles in a procedural mesh. The problem is even though you can index large positions in your vertex buffer, your triangle buffer still has a smaller limit, because all of the ways to assign data to your mesh instance have limited sizes in one way or another. Most of the managed array overloads only deal with int/ushort arrays (no uint?). So you’re forced to use mesh.SetIndices, but even there Length in the method signature only accepts int, no uint.
An index buffer with 31bit uints, as currently allowed, is 8GB in Size. Lets assume your vertex buffer contains a triangle list with position, normal, tangent and uv → 96GB. So to use the currently possible limit of 31 bit with a ‘standard’ vertex buffer format you need 104GB on your GPU and temporarily on your CPU.
Do you have an actual problem you need to solve regarding this?
You’re assuming that this particular mesh needs all of that data, when it may not necessarily need anything more than the vertices and triangle indices, still large but not nearly as large
If you keep being vague with your answers, people are going to be vague with theirs or offer no help at all… Those people asked you those questions in order to explore additional solutions to your ever so mysterious problem.