Import with custom profile fields

I’m struggling with the import function… after a slow start, I can now manage to import users OK *except* for their custom profile fields. Having looked around the forums, I think I have gleaned the answer, but want to double-check:

– it seems that there are some ‘standard’ fields that are always required. I couldn’t even get some of the basics to work until I included fields like Website and Custom Capabilities… are these mandatory ? (and where is this written)

– there are several standard fields that I have omitted from the CSV, as these are neither available nor relevant in the existing system. Is this what is preventing the Custom Profile Fields from importing? It seems strange that one can omit fields in a normal import, but need to have all fields present to get the Custom Profile Fields working

… or am I missing something?