English is not my native language, and although English not always is a very precise and logical technical language, I think , in this case, the logical explanation is the correct one:
Seat ring: ring forming a seat, eg. the part of the valve where the closing element 'sits down' or fit against to close the valve.
The moving gate is here the closing (moving) element. A part forming the seat can by definition never be mounted on the closing (moving) element, only on the counterpart (on the valve body).
NB! For all valves (part of) the sealing is mounted on the moving closing element (or originates from prepared surface of..), and is in some cases held in place on the closing element by a ring (clamping ring). This is then often given names as seat sealing or seat sealing clamping ring or similar. This must be understood as a sealing construction FOR the seat, and not confused with the seat or seat ring.
With this in mind your valve body seat ring is the ring forming the seat, mounted to the valve body.
The other part is a bit unclear worded for me.
Disc facing valve ring? Disc facing ring? Unclear wording here! Could either be a ring mounted on the disc (gate- or closing element?) face (surface) to perhaps be a counterpart to the seat ring? Or be another way of describing the seat ring itself, facing the disc (gate?).
The most likely is perhaps a try to translate a foreign language datasheet to english, with a bit confusing result.
