Custom Fields are a feature of the Advanced Plan. Custom Fields allow you to add fields to your Project that capture custom information not included in the standard Project fields.
These Custom Fields can be Checkboxes, Dates, Dropdowns, Numbers, Percentages, Ratings, Staff, or Text based fields which you can then label however you want. While all fields can be included in reports, Drop Down based custom fields can also be used as filters.
To create and edit Custom Fields, just go to your Settings and select Custom Fields from the left-hand menu. You can then click "+ New" to add a new field, or click the ellipsis next to any existing Custom Field to edit it.