From the "UC_Surface_ReferenceManual_EN_04042019", section 2.5 Filter DCAs: "A Filter DCA can contain any combination of the available input channels and effects returns". This is 64 input channels and 8 effect returns as well as Aux In 1, Aux In 2 and Tape In (Digital In).
It is not possible to add flex mixes (aux, subgroup, matrix) to DCA groups.