The value for the Immediate Alert Service is defined to be uint8. However, if I try to pass a value above 2, it doesn't pass the higher value.
I think the immediate alert only works with 3 alert levels. So 0-2 it is. Best to look up the official spec for this service if you want to be sure.
The spec. does imply that only three levels are allowed.
The spec allows only three levels, but you can implement a custom profile based alert notification in which you could have more alert levels.