In every the Contact record, there is a section called Contact Types (accessible on the Basic tab). This is where you indicate what type of contact someone is, and how they interact with your organization. Very common examples of the Contact Types we often see are things like "Board Member," "Client," etc. Contact Types are customizable, so however you want to label and categorize the contacts in your own database, you can!
In addition, Sumac tracks all data related to a contact (i.e. donations they make, hours they've volunteered, etc.). Tracking this data allows you to build a bigger picture of the interactions you've had with a contact and understand their role in your organization. For example, if you have a Board Member in your database, you could assign a Contact Type of "Board Member" to this contact in their contact record, record details of the donation(s) they've made in donation records, and record the details of hours they've volunteered in Time Dockets. This gives you a full picture of all the different interactions you've had with this contact, and their various roles with your organization (i.e. board member, donor, and volunteer).