The client register and the login vault help agencies keep track of which sites belong to which client and store site logins securely.
The client register
Under Clients you add clients with a company name, contact person, e-mail, phone and notes. Each client shows how many sites are linked and a total of the monthly price. You can link a site to a client here or from the site itself under Client & login. The client register is part of the Agency and Partner plans.
The per-site login vault
On a site you can store the site's login URL, username and password - encrypted - whether or not the site belongs to a client. A technician can reveal the details when fixing something: click to reveal the password or copy it. The details are visible only to your own logins and are never shared via the API, widgets, or with anyone else. The vault is a Pro feature (the feature key sites.login_vault).
Unlinking does not wipe the login
Turning the client link off, or deleting a client, does not wipe the site's stored login. The sites stay, they are just unlinked, and whatever is in the vault is kept.