Actually, I was mistaken, this did not work. I didn't look carefully enough. It so happened that the few types displayed in the lines allotted to the combo box, showed only those items alphabetically associated with that category, from the value that was already present in the Type combo box. I had played with an asset that had already been classified, and re-selected the category to cause the Requery, but when I checked further, no actual filtering occurred. I now realize that I hadn't done the expression as you specified, so I redid sql, generated from the Design View, as follows, but it errors on the Forms expression in the Where clause.
Code:
SELECT Asset_Types.*, Asset_Types.Asset_Type, Asset_Types.Asset_Cat, Asset_Categories.Asset_Category
FROM Asset_Types, Asset_Categories
WHERE (((Asset_Types.Asset_Cat)=[Forms]![Assets]![Category]))
ORDER BY Asset_Types.Asset_Type;
Perhaps I should restate, that the Assets form is in a NavigationSubForm, with the tab named "Assets".
I got to thinking that there may be a drawback in this approach anyway. For when one might want to change a type value for an existing category value, the refresh will not happen, to filter the list.