Time-limited access a customer has bought, or that you have granted by hand.
Unlike subscriptions, these can be granted manually
You can issue a membership for any email address with no order behind it at all — a trial customer, a goodwill gesture, a sale made off the site.
The link is by email, not by user account. If the customer later registers under a different address they will not see this access; the address has to match exactly.
An empty end date means permanent
A membership with no end date never expires. If you mean a fixed period, set a date.
"Extend" sets a new date, it does not add
When extending, the date you enter is the new end date; it is not added to the existing one. For one more month, work out the final date yourself.
An expired membership whose end date is moved into the future becomes active again.
Expiry happens on its own
A membership whose date has passed moves to expired by itself — there is nothing to close by hand. The customer's access ends with that status change.
A membership from a subscription is not independent
A membership created by a subscription is revoked when that subscription is cancelled. If access is meant to survive independently of the recurring payment, issue a separate manual membership.