Hieracium x rubrum is one of the forms, which hybrids between Hieracium aurantiacum and Hieracium pilosella can adopt, namely the ones closer at Hieracium aurantiacum: relatively high growing and with an inflorescence consisting of several flower heads with short peduncles.
At genus Hieracium there is the tradition to name (stabilized) hybrids resp. hybrid populations differently, depending on to which of the parents they are approaching. Forms of the same hybrid combination which tend rather to Hieracium pilosella have Hieracium x stoloniflorum as hybrid name.