It's purely a convention to use terms of the form "key - value" for 2-tuples here (or '-'(key, value) in funtional/canonical notation which can be used as well). minus is just used because it's already predeclared as an infix operator, and indeed ','(key, value) could be used as well but comma is also used as argument separator and for conjunctions in clause bodies and thus tends to be avoided. You also can see '=' being used for the same thing eg.

    [ key = value, ...]

(for example, as used for representing attributes by SGML/XML parsing libs for SWI, Quintus/SICStus, and others), not to be confused with '=' being interpreted as unification operation in goals/clause bodies.

If you think about it, the simplest convention in Prolog to represent "assignment" of a value to a symbol (not a variable) would be

    key(value).

That is, to use the "key" atom as functor itself, rather than use functors/operators in ad-hoc ways. This is exactly what Quantum Prolog can do (optionally, and in addition to ISO Prolog's conventions).

Specifically, if you have a list of fact-like terms

    L = [ p(1), q(2), r(what(ever)) ]

then Quantum Prolog can answer queries against such term list, just like answering against the global default database eg.

    call(L ?- q(X))

binds

    X = 2

and would also bind additional values for q(X) on backtracking if the term list contained any. This is a natural extension to regular querying in Prolog because a term list [a, b] in Prolog's square bracket notation is just syntactic sugar for using the dot operator

    '.'(a, '.'(b, []))

and a Prolog program is syntactically just a list of clause terms.

In the container planning demo on the Quantum Prolog site [1], this feature is used for backtracking over (un)loading and travelling actions which would normally change state via destructive assert and retract calls and hence not allow backtracking to search for optimal sequences of actions.

[1]: https://quantumprolog.sgml.net/container-planning-demo/part2...