This thread is a collected effort from users to dust up attributes that can be called via USER_DATA nodes in shading - for 3DS MAX.
mtlID - Material ID index
uv_1 - UV channel 1, prefix channel number with "uv_"
paLife - Pflow Particles Life
wireColor - transfers a mesh's wire color to shading directly
root_colors - Hair&Fur, pr. strand color based on a control map
gBufID - The Object ID, assigned in the legacy Object Properties
If you find, or have knowledge about attributes and how to enable them, make a post in this thread and I will update the main post so the list grows over time.