Model 901: change in the ownership, royal rights (e.g. usufruct), administrative concession, acquisition/change of quotas of un “bien inmueble” (urban real estate, whether dwelling, flat, garage…) of an urban nature. It’s the change of titularidad catastral or cadastral ownership.
Model 902: change in the “nature” of the urban property:
new build, extension, rehabilitation, reconstruction.
http://www.catastro.meh.es/esp/procedimientos1.asp#menu2