Interval key in
/System/Library/LaunchDaemons/com.apple.periodic-daily.plist from my MacOS 10.13.6. But I fail to find what
Interval is by googling. Even
man launchd.plist does not have this key. Thanks.
The difference between StartInterval and ThrottleInterval is that StartInterval means that the program should be started after x seconds (more or less precisely), whereas ThrottleInterval means that the program should NOT be started again less than x seconds after it was last started.
So StartInterval is used to schedule something to run every X seconds, where ThrottleInterval is used when you schedule something to run based on other criteria, but don't want it to run too often.
"Interval" does not seem to be a valid key - it is not in the man page, as you describe, but nor is it in the source code released by Apple. Either it is a mistype for "StartInterval", or it seems Apple has included "Interval" as an alias for "StartInterval" in their own builds.