The attribute substitution category is assigned to a device or junction that can substitute or "swap" the phasing of a downstream line, it must also have a network attribute that is configured for attribute substitution defined for it, and any features that are substituting phases must have that attribute populated. If you have a double dead-end pole top assembly where the B and C phases are swapped, you'd use attribute substitution.
A subnetwork tap is a network category assigned to a feature that allows the feature to not restrict phasing on the downstream tap line. If you have a B-phase line tapped off a 3-phase overhead line, you would place a B-phase overhead tap (which has the subnetwork tap network category assigned to it at the intersection of the two lines. Because it is a subnetwork tap it would allow the B-phase line to be energized without restricting the phasing of the mainline to also be 3-phase. If you were to place a 3-phase dead end on the mainline, it would restrict all the downstream lines to be B-phase.