In the chain of development, Yesod of Atik Yomin ends in the chest (Tiferet) of Arich Anpin; as clothed, it ends in Yesod of A"A.
Applied directly: Yesod of Atik ends in Tiferet of A"A (chain) and in Yesod of A"A (clothing). Same fact, two modes.
Op. 110 opens the Abba and Imma unit by applying Op. 109's two-modes rule directly. The Atik-A"A pair becomes the paradigm case: same structural fact, two valid descriptions. Op. 110 is the worked example that the rest of the unit (Op. 111–114) operates under.
In the chain of development: Atik's operational radiation extends downward into A"A; Yesod of Atik (the Atik level's generative Sefirah) reaches the operational endpoint of Tiferet of A"A (A"A's middle anatomical level — the heart, the chest). This is the chain-mode endpoint.
As clothed: Atik wraps A"A; Atik's interior fits into A"A's anatomy; Yesod of Atik becomes the Yesod of A"A (the same anatomical position one level deeper). This is the clothing-mode endpoint.
The two endpoints are different anatomical positions. Tiferet of A"A is the middle of A"A's body; Yesod of A"A is the foundation. But both are correct descriptions of where Atik's Yesod ends — once in chain-mode, once in clothing-mode. The same Atik-Yesod, two structural-anatomical endpoints.
This paradigm runs through the rest of the named-Partzuf walk. When Op. 113 says Abba-Imma coupling is constant; when Op. 116 says Zeir Anpin (Z"A)'s Mental Powers descend from Abba and Imma; when Op. 138 says the cosmic-operational mechanism runs through Z"A and Nukva — all of these are structurally specific instances of Op. 109's rule applied to specific Partzuf-pairs.