In some cases you want to check and see what data type a variable holds in GameMaker Studio 2 and that's when you would use this function. It returns true or false depending on whether the value is a 4 value vector or not. For further information on data types see here.
Argument Description n The argument to check.
show_debug_message("Not a 4 value vector!");
The above code checks the variable "val" to see if it contains a vec4 and if it is not then it shows a message in the debug console.