The category drop down allows you to select which industry or category your job is for. This is then passed to the job boards that support this type of sorting so they can place your job in the corresponding category on their site.
What if I don't select a category?
The category field is not required so you may leave it blank if you wish. If you leave it blank, the job will still post to the job boards, it just won't be contained within a specific category.
Why would I want to pick a category?
Selecting a category will place the job in that category on the job boards that support it. When a candidate does a filter or a search on these job boards, your job can then appear in their category search.
Can I add categories to this list?
The category list is created by combining popular options listed by the world's most popular job boards and is not able to be customized.
If you'd like to create your own customized category list, you can consider creating a custom field.