It is possible to define Icon & Icon in map for every item category. When you create the sub-categories then it is recommended to NOT define the icons for them and it will be explained later why.
There is no problem when 1 item is assigned to 1 item category or to more sub-categories (of the same parent) but with no icons defined. Because in such case there is no problem to decide which icon to use.
But when you assign 1 item to more item categories (or to sub-categories with different icons) then we have to decide which icons to take. Our directory templates are designed by following way the icons form 1st category (or sub-category) in alphabetical order are taken and this mustn’t be always suitable for you.
Please take all this into account when you design your item-item category structure!