Map on class of all finite posets coming from maximal sized antichains Let $P$ be a finite poset. Let $\mathcal{A}$ denote the set of antichains of $P$. Equip $\mathcal{A}$ with a partial order $\preceq$ whereby $X \preceq Y$ means for all $x \in X$ there exists $y \in Y$ such that $x \leq_P y$. Then it is easy to see that $(\mathcal{A},\preceq)$ is isomorphic to $J(P)$, the distributive lattice of order ideals of $P$ (just send an antichain to the order ideal it generates).
Let $\mathcal{A}_{\mathrm{max}} := \{A \in \mathcal{A}\colon \forall_{B\in\mathcal{A}}\#B\le\#A\}$. Then it is known that $\mathcal{A}_{\mathrm{max}}$ is a sublattice of $\mathcal{A}$ with respect to the partial order $\preceq$ (see the paper of Freese linked to below). In particular, $\mathcal{A}_{\mathrm{max}}$ is a distributive lattice, and hence by Birkhoff's Fundamental Theorem of Finite Distributive Lattices, has the form $\mathcal{A}_{\mathrm{max}}=J(P')$ for some other (unique up to isomorphism) finite poset $P'$, namely the induced subposet of join-irreducible elements.
The map $P \mapsto P'$ defines a self-map on the class of all finite posets. This map is far from injective: for instance any poset with a unique antichain of maximal cardinality (for instance $P=n\underline{\mathbf{1}}$ an antichain) is sent to the empty poset. On the other hand, for $P=\underline{\mathbf{n}}$ a chain, we have $P' = \underline{\mathbf{n-1}}$. Also, experimentally it seems that with $P = \underline{\mathbf{a}} \times \underline{\mathbf{b}}$ a product of chains, with $a \geq b$, we have $P' = \underline{\mathbf{a-b}} \times \underline{\mathbf{b}}$. (And it looks like similar behavior may propagate to product of three chains and beyond.)
Broad question: How can this map $P \mapsto P'$ be understood? Is there a "simpler" description than the one I have given? Are there other families of posets on which it exhibits interesting behavior? (EDIT: Other questions along these lines: Is the map surjective?, Is $P'$ always isomorphic to a subposet of $P$?, et cetera.)
Specific question: Is it true that this map has $\underline{\mathbf{a}} \times \underline{\mathbf{b}} \mapsto \underline{\mathbf{a-b}} \times \underline{\mathbf{b}}$? What about products of many chains?
Freese, Ralph, An application of Dilworth’s lattice of maximal antichains, Discrete Math. 7, 107-109 (1974). ZBL0271.05011.
 A: The references given by Richard Stanley answer most of my array of questions.
In particular, Koh showed that the map $P \mapsto P'$ is surjective in "On the lattice of maximum-sized antichains of a finite poset", and gave some more information about the fibers of this map in "Maximum-sized antichains in minimal posets" (both linked below).
Also, in Engel's MIT PhD thesis (available online), she computes $P'$ for various posets $P$, including the case of the product of three chains; she describes (in Section 3.3) the distributive lattice in question as the lattice of Semistandard Young Tableaux of rectangular shape $c^r$ with entries at most $n$ with coordinate-wise order, but it is easy to see that this is the same as $J(\underline{\mathbf{c}} \times \underline{\mathbf{r}}\times\underline{\mathbf{n-r}})$. (Note that she views the map in question as being of the form $J(P)\to J(P')$, but of course by the Fundamental Theorem of Finite Distributive Lattices, this is no different than $P\to P'$.)
Koh, K. M., Maximum-sized antichains in minimal posets, Algebra Univers. 20, 217-228 (1985). ZBL0602.06001.described some
Koh, K. M., On the lattice of maximum-sized antichains of a finite poset, Algebra Univers. 17, 73-86 (1983). ZBL0524.06003.
