If $K$ is a bicategory, then a morphism $f \colon a \to b$ is called a map if it has a right adjoint $f^* \colon b \to a$. (This is in slight contrast to the common usage of “map” to denote simply a morphism in any category.)
The bicategory $Map K$ is the locally full sub-2-category of $K$ determined by the maps.
In the bicategory Rel of sets and relations, a relation is a map if and only if it is the graph of a function. Consequently, $Map Rel$ is equivalent to Set.
Similarly, if $C$ is a category with finite limits, then there is a bicategory $Span C$ of spans in $C$. The bicategory $Map Span C$ is equivalent to $C$.
In the bicategory Prof of categories and profunctors (perhaps enriched), if $B$ is a Cauchy complete category, then a profunctor $A\to B$ is a map if and only if it is represented by a functor $A\to B$. If $B$ is not Cauchy complete, then maps $A\to B$ correspond to functors from $A$ to the Cauchy completion of $B$.
If every map in $K$ is comonadic? and $Map K$ has a terminal object, then $Map K$ is equivalent to a $1$-category. If in addition $K$ is a cartesian bicategory and every comonad in $K$ has an Eilenberg--Moore object, then $K$ is biequivalent to $Span Map K$, $Map K$ having finite limits. The converse is true if pullback squares in $Map K$ satisfy the Beck–Chevalley condition in $K$, i.e. if their mates are invertible (see [LWW10]).
$Map K$ is a regular category if and only if $K$ is a unitary tabular allegory, equivalently a bicategory of relations in which every coreflexive morphism? splits. In that case $Rel Map K \simeq K$.
Similarly, $Map K$ is a topos if and only if $K$ is a unitary tabular power allegory.
A 2-category equipped with proarrows is, by definition, a bijective-on-objects pseudofunctor $K\to M$ such that the image of every arrow in $K$ is a map in $M$. Equivalently, therefore, it is a bijective-on-objects pseudofunctor $K\to Map M$.
Hence the inclusion $Map M \to M$ is the “universal” proarrow equipment that can be constructed with a given bicategory $M$ as its bicategory of proarrows. More precisely, there is a forgetful functor from $Equip$ to $Bicat$ which remembers only the bicategory $M$ of proarrows, and the assignment of $M$ to $Map M \to M$ is its right adjoint.
Mike Shulman: This is obviously morally true, but I can’t be bothered right now to check which 1-, 2-, or 3-categories of equipments and bicategories one has to use to make it precisely correct.
A lot of work in bicategories that makes use of maps could easily be reformulated in a proarrow equipment, and conversely. Thus, it is to some extent a question of aesthetics which is preferred. One advantage of proarrow equipments is they can distinguish between a category and its Cauchy completion (as objects of Prof), while maps in bicategories are perhaps simpler in some ways.