Categories representation in the default X-Cart 5 layout is rather straightforward - a list of categories in the left-hand column on every page. The add-on Flyout Categories Menu extends the default representation with a vertical drop-down menu for the categories list.
If necessary, the default categories representation can be transformed into horizontal menus as described in Categories Representation on the Storefront. But if you do not want to play with the default menus to configure them this way and need a more robust solution that will add extra facilities to the store or change the categories representation drastically, use one of the following add-ons:
Horizontal Flyout Categories Menu - Moves the categories list to the horizontal bar with a drop-down menu.
Multilevel Primary Menu - Adds the multiple drop-down menu functionality for the header tab menu.
Tree-like Categories Menu - Transforms the default fly-out category structure into a tree structure.