“Category” helps you classify issues into general groups, which is helpful as you can use them as an additional filter for Search conditions.
Multiple categories can be added for each Project by both Administrators and Users without any restrictions. You can add a new category while creating a task or issue (on the issue page), or go to the Project Settings page to manage categories.
On Project Homepage, you can view the overall number of issues for each category. This also provides an overview of your current task progress per category.
Categories can be defined in different ways according to project needs and user preferences. Feel free to share with us how you create your Categories. 
Happy Collaborating!