How a showcase is owned with no account, where its key lives, and how to back it up and restore it
Your showcase has no account and no password. It belongs to a key, and every change you publish is signed with it. Whoever holds the key owns the showcase.
Where the key lives
The app keeps each showcase's key in your iCloud Keychain. Your iPhone, iPad and Mac on the same Apple ID share it, so a showcase made on one appears on the others by itself. The Mac never makes a key of its own for a showcase it didn't create: it waits for the real one to arrive.
Backing up: the recovery key
For anything iCloud Keychain doesn't cover, such as a new Apple ID, a staff member's phone or a client's device, there is the recovery key: one line of text starting showcase1:, also shown as a QR code.
Open iCloud (at the foot of the sidebar, or of the list on the iPhone), then Show Recovery Key. It shows the key as text and as a QR code: Copy it, Save File…, or Share… it to a password manager, Files or a printer.
The iCloud card also says whether this showcase's key is in your iCloud Keychain and how many showcases have a key on this device. Refresh looks for keys from your other devices and says what it found.
Keep it somewhere safe, such as a password manager or printed in the office. Whoever has it owns your showcase, and can also read the values you keep only in the app. See Only in this app (LS-5).
Adding a showcase with its recovery key
Tap or click the showcase's name, Manage Showcases…, then Add with a Recovery Key…. Paste the key, or Open a Key File…, or on the iPhone and iPad Scan Its QR Code… with the camera, pointed at the other device's Show Recovery Key. Then Add Showcase.
Paste it exactly as it was sent. If the app says That isn't a Showcaser recovery key., part of it is missing.
Sharing a showcase
To manage a client's showcase, ask them to send you its recovery key from the iCloud card (Show Recovery Key). You will both be able to manage it, and each of you sees the other's changes. See When two devices edit at once (LS-7).
The same manual ships inside the app. This section as Markdown