I heard from a colleague that project classification needs to be done to be compliant to the Project planning processes. Which Project Planning Specific Practice (SP) refers to project classification?
I am not sure what you mean by project classification. Do you mean application domain? Or project size? Or something else entirely?
My best advice to you is not to listen to colleagues who may not be CMMI experts, but instead work with a CMMI consultant or SEI-certified Lead Appraiser. They will provide you with the best information and CMMI interpretations.
The CMMI does not state that you must classify projects for Project Planning (PP). That is why you are having difficulty determining the applicable Specific Practice.