No — they are three separate things:

1. Membership Plans (Marketing > Membership Plans): named tiers you assign to customers, used to limit coupons to a group of customers.
2. Subscription Memberships / Account Members / Members: the feature that lets one customer (the account manager) manage several linked customer accounts (family members, employees, clients), including paying for their subscriptions. Admins see it on the customer profile's Members tab; customers see a Members link in their dashboard; the admin module list calls it Subscription Memberships; the report is the Memberships Report. This part is gated: the Members tab, the customer-facing Members link, and the Memberships Report only show up when the Subscription Memberships module is enabled or the account is on the Base, Pro or Enterprise plan.
3. Membership ID: a free-text field on the customer profile (labeled Membership ID, up to 50 characters) for storing an external reference number, e.g. a gym membership number. It can also be turned on as an optional or required field on the customer registration form (Business: Website settings, "Fields to ask on Register" / "Fields mandatory on Register"). It is not linked to either feature above.

This knowledge base covers all three, but they do not depend on one another.