Environment variables
Tokens, connection strings, and anything else that should not be in your code.
Setting a value
Values are encrypted before storage. Setting one restarts the app by default, because a running process does not pick up a new value on its own.
Why you cannot read them back
There is no way to retrieve a stored value — through the panel, through the API, or through a connected agent. Only replacement.
This is a deliberate limit rather than a missing feature. An agent with access to your account is a useful thing; an agent that can read every secret you own out of that account is a liability. Write-only keeps the first without the second.
Names we recognise
Any other name works too — these are just the ones our templates expect. Your code decides what it reads.
There is no list to register a name on. A variable your app has never had before is applied exactly like one it already uses, on a running app, without redeploying.