Within Titan, each and every single item which you program has a number - simply an integer number, which you, within reason, can assign freely. In most cases, this is either not conciously used at all, or used in a user-specific system (e.g. group #1 might be the front truss or a specific group of fixtures…). The userNumber can be used to refer to a specific item. Unlike the titanId which is unique in a global scope (i.e. per show there can only be one item with a specific titanId), the userNumber is unique only in its context (e.g. fixtures, groups, palettes, playbacks, macros etc.): there can only be one fixture with the number 1 - any subsequently patched fixture will be given another userNumber. But there may also exist group 1, palette 1, or playback 1.

userNumber is a positive integer value.

Usually the userNumber is passed like this:

userNumber:2

Technically, this syntax looks like https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/named-and-optional-arguments: you have to name the argument 'userNumber', and after a colon give the value.

ASSUMPTION

It is a reasonable assumption that AcwUserNumber also refers to userNumber, but in the context of the value being given in the appropriate menu.