https://support.appcoll.com/matters/add-custom-user-defined-fields-to-matters
You can add custom fields in the matters module - up to 16 string fields and 4 date fields - to store information associated with a matter that is unique to your account. These custom fields appear in matter detail pages around the middle of the page.
When setup, these fields have identical functionality to the built-in fields. They can be added as columns to the main matters page and used in filters. They can also be imported and exported via CSV spreadsheets.