When you apply a category to an entity, the category controls what the resources working on the entity will see. The following things can be customized using categories:
Page elements
All entities have page elements and body components that can be hidden on items associated with the category. A visual timeline, for example, is optional on tickets, tasks, and opportunities. Refer to Main body.
Fields and how they are grouped
Using categories, you can manage which fields are visible on the New, Detail, and Edit pages of an entity. Are there fields your company just doesn't use? You can use the category to hide fields that are not necessary, making it easier for your users to get to the important information. Most, though not all, fields can be hidden, and you can also decide how you would like to group fields into custom sections. Refer to Sections & Fields.
Field properties
Each category allows you to configure different field properties. Do you want to set a default value? Do you want to make a field required for a workflow to avoid later questions from another department? Do you want to limit the options users can select from a List type field or a database table? Refer to Configuring field properties.
Insights
Categories also allow you to determine which insights, if any, will provide information from entities related to the item. Do you want the opportunity to provide some details about the related proposal project? Refer to The Insights tab.