They are the motives proper to the specialty as such, but not to the specialty as participant in a wider context of motives.
From page 1329: "Any specialized activity participates in a larger unit of action."
Burke seems to be reiterating the social and political web that Nietzsche suggests throughout On Truth and Lies in a Nonmoral Sense. The fictions we come to believe as truth must be taken into consideration as part of a larger network of relationships and systems that are at once predetermined and unstable.