structure is a theoretical variable type that is still in process of being defined.

It is intended to hold arbitrary complex data structures as you may be creating in any application and may want to share with any strictly compatible application.

The requirement is not to be extensible, not to support inheritance, not to even be intelligible by anyone else but the application creating this. Of course you could use any format then, but still it may prove useful to have a standardization here.

JSON is the most popular choice of syntax for this sort of problem, but consider that JSON comes with a lot of overhead which is not necessary in PSYC. We should propose a more efficient syntax than that.

Structures are not to be confused with lists and tables, even though they may choose to make use of them. And of course you can always choose to encode your structures in form of a set of variables of various types, that carry all the information you need.