zulip/help/include/manual-billing-intro.md

With automatic billing, you automatically purchase a Zulip license for each user in your organization at the start of each billing period (month or year). [Deactivating a user][deactivate-user] frees up their license for reuse. Additional licenses are purchased automatically as needed.

With manual billing, you choose and pay for a preset user limit. If he limit is reached, no more users can join until licenses are manually added.

In general, selecting automatic billing is highly recommended unless you have a specific reason to do otherwise.