Membership ID is a free-text reference (up to 50 characters) stored on the customer profile — for example a gym, club or corporate membership number. Admins can fill it in under Customers > edit customer > Membership ID. When it is set, the customer sees it as #value next to the title of their profile page in the dashboard.

It is purely informational: it does not link to Membership Plans, does not grant discounts and is not used by account members.