The Arguments tab in Setup > Functions & Variables allows you to further customize the arguments you pass to your scripts.
Default
As the window informs you, you can optionally define a second layer of defaults to function arguments.
Within the argument hint, a double colon ::= means the default comes from DIISIS. A := means it comes from the script itself.
When writing functions, if a default argument is before a mandatory argument, use * to access the default.
Stringkit Typing
Additionally, you can type any String argument as Stringkit.
If you do this, the argument needs to be one of the options of the stringkit. e.g. foo(bar:character) and character is a stringkit. You can permit multiple stringkit as types. Then, the argument will be valid as long as the argument is an option in any one of the stringkits.