what is a dependant relative for CGT?

I know a dependant relative must be rent free, and be a blood relative ( or adopted etc) but what is the definition of the 'dependant' bit- do they have to be infirm/elderly or can it be say, a student? Or even a bit of a 'waster' adult child? Is there a definition somewhere or is each case judged on its own merits?

section 604(11) defines a dependent relative (for the purposes of the PPR relief) as "
a relative of the individual, or of the wife or husband of the individual, who is incapacitated by old age or infirmity from maintaining himself or herself, or the the widowed father or widowed mother (whether or not he or she is so incapacitated) of the individual or of the wife or husband of the individual".

So, no waster adult children included.

Thanks Nige, so based on that then even a student wouldn't qualify.