Its a special cnc that scales everything in the Z axis to 1/2 of full size
Theoretically, a 2.5d cam is incapable of generating a ramping move that could have unique endpoints in all 3 axis. It could produce XY moves, followed by Z moves, all movements being produced by motors on the machine. If you imagine drilling holes with a manually operated quill, on a cnc that only positioned the table in X and Y, that would be 2d. Add a motor to the Z axis to pull the quill for you, you've got a .5d capability.
This definition may be too limited now, because modern cnc's are much more capable. 3 axis interpolation is easy to come by on the machine controller. So perhaps a more modern version of 2.5d would be cam software that can read only wireframe geometry information versus software that can create toolpaths from electronically modelled 3d surfaces.