A relative clause with its verb in the subjunctive describes a kind of person or thing ("the sort who would …"), or gives a reason or a purpose.
Characteristic: after nemo est qui, nihil est quod, sunt qui, is qui, dignus qui, the subjunctive means "the kind who would, who could": nemo sit, qui prohibere te possit "there is no one who can stop you".
Cause: ut qui, quippe qui, utpote qui + subjunctive "as one who, since he": ut qui … possis discedere "as one who may leave". tanquam qui + subjunctive "as one who, as if": tanquam qui jam statim moriturus sis "as one who is about to die".
Purpose: qui = ut is, "(in order) to": scopum, ad quem … dirigant "a target at which to aim".
A relative clause may also be subjunctive only because it stands inside reported speech or another subjunctive clause (see the subjunctive in reported speech).