While ordering your menu’s main categories by sales volume is preferred, sub-categories are best listed alphabetically. This is simply a matter of practicality.
Sub-categories are usually listed vertically and can contain a pretty large number of items. Ordering a long vertical list randomly is no short of an act of torture towards your would-be customers. It would certainly lead to a lot of frustration and cost you conversions.
An alphabetically ordered list is ideal as it can be scanned quickly and your customers are able to know which letters to look out for instead of having to read the entire thing. It’s also quite easy to implement from a design perspective as programming software to arrange items in alphabetical order is comparatively easier than other sorting options which often require mathematical operations.
Also take care that you don’t end up listing 100 sub-categories. According to Baymard, most subjects feel overwhelmed when faced with more than 10 sub-category items to choose from. A good way to deal with this is by dividing your sub-categories into easily manageable sub-sub-categories. You should keep your sub-category hierarchies logical, easy to understand, and easy to scan. These can be subdivided up to three levels.
Let’s take the example of a fashion retailer. You can have men’s clothes, women’s clothes, and unisex clothes as your parent categories. Under the men’s category you can have formal wear, casual wear, and underwear as your primary sub-categories. Under the casual wear section you can have pants, shorts, shirts, jackets, and hats as your secondary sub-categories. Your hats can be further subdivided into sombreros, fedoras, newsboy caps, and baseball hats to form tertiary sub-categories.
It’s also a good idea to make your sub-category items clickable. 14% of sites make the sad mistake of listing them as just plain text labels.