Open the “Inventory item” reference selector edit window (double-click), then go the “Selectable Objects” tab. At the bottom, you will be able to see a property called “Constrained by” which you can use to filter the list of selectable “Inventory items” based on the selected “Category”.
You can refer to this URL for more details: https://docs.mendix.com/refguide/reference-selector
It might be useful to go through the whole documentation page; however, your section of interest in the page is titled: “Constrained By”. It says the following: “A reference selector can be constrained by one or more paths. This is typically used to make one reference selector dependent on another. For example, in page where you can edit an order line, a product selector can be constrained by a category selector. After selecting a category, the product selector is constrained by this category and shows only products in the category.”. There are also figures and there is a more detailed description of the example which might help you understand how to use it better if you refer to the URL mentioned above.
NOTE: Try not to be too generous while using the “Constrained by” property because I have come across performance issues when there are more than 4 constraining paths selected and a large number of records is available in your tables. Of course, this does not seem to be the case here (since you are only planning to constrain it by “Category” for now) but I thought I should mention this just in case you face this issue in the future.
Thank you very much!