Note: if compatibility version is not listed the one you need, contact me and i will let you know if extension works with your version.
By default in OpenCart when you go to assign a category to a product (in admin) you must remember what is the category name in order to start typing it and later (select it from autocomplete 5 suggestion list) what category the product must be added to.
That is all ok if you do remember all the categories you have added:). But what if there is so many similar subcategories that you not able to remember ? Or even if you remember but the drop down list shows just selection of 5 matched categories. In this scenario you must go to category page and check the category name.
Later get back to edit product and type the correct category name in order to add it to product, or you must manually type all subcategory name if you have almost alike subcategories under main categories.
This extension will change the way you assign categories to your products, from now you will see for the ease of usage all categories sorted alphabetically, so you can simply scroll and select the categories you want to add the product to.
PS: if you looking the very same functionality for Admin Product And Category Filter Select List