This is a misunderstanding.

You can not save an expression category “by itself” as a file.

What you can do:

1) You can create a new expression category in a Finale document.

You do so by duplicating an existing category, and editing the duplicate.

The new category will be saved in the Finale document.

2) You can save a library of expressions.

Each expression in the library belongs to a category which gets saved with the expression.

When you load that library into a Finale document, not only the expressions will get loaded into the document, but also their categories.

3) When you paste or insert copied music into a Finale document, all expressions (and their categories) from the copied music go into the Finale document as well.

In Short:

You can not separate a category by itself (= without an expression).

A category always is “attached” to an expression.

Since libraries are cumulative, I wondered whether taking each expression category in turn and deleting all the items in every other category before saving the library (under a new name) would allow one to add each category in turn to a new Document Without Libraries. I figure it would be very cumbersome to create but thought it might work if nothing else did.

One thing that is often overlooked, as I found out : if one creates a new category (from a duplicate) and if one wants to save a library and load it into another file, the parameters of the new category should be slightly different than the one it duplicates (a slightly different music or text font attribute, positioning, ...).

If it is 100 % identical, it won't work. The category will not load. My experience.