Rockwell, as is there way, uses different terminology from the reset the world. So, while any computer programmer or controls engineer for a different platform would call it declaring a variable, Rockwell calls it creating a tag. So, your tag lists are Rockwell's version of a declaration of variables. I'm kind of curious what they call it in the Yaskawa world.
In a lot of platforms (including everything based on Codesys), you can define the variables in the Structured Text standard format like this:
MyBool : BOOL;
MyInt : INT:=5; (*I initialize this INT to 5 on startup*)
MyArray : ARRAY[0..9] OF INT; (*an array of ints*)