Reflexive Sequence is a property of Relating: relating can take its own activity as subject, producing indefinitely deeper layers of self-reference.
Reflexive Sequence is the first source of depth in the derivation. Before Relating, the derivation had acts and conditions, but no capacity for self-reference. Relating introduces self-reference because its own activity — sustaining the interplay of Including and Excluding — is itself something that can be related to. Once this capacity exists, there is no natural stopping point: each layer of self-relation is as legitimate as the last.
Reflexive Sequence gives depth (going inward, layer by layer). Composition, a property of Relational Form, gives the complementary dimension of breadth (forms placed alongside each other).