Categories are used to associate families to a group. Doing so can make the selection process easier for your families, as families can be found both under a general alphabetical listing as well as the categories they’re in. Likewise, using categories can increase your fundraising since families can easily opt to send to a whole category with the click of a button. This is true especially if you assign a discount price to a category to encourage families to select a whole category (see Campaign Settings, Categories).
Categories need to be created and families associated with these before the start date of your campaign. Association with categories, or changing associations cannot be done after the start date.