Ullabella is a variable. You can use pretty much anything you like as a variable name, as long as it's not the name of anything in the game (like M16 for example). You can use each name only once in each mission - if you have a variable Ullabella you can't have a tank with the same name.
In this case the variable is a boolean variable - it can be either true or false. You define which it is like this
Ullabella=true
in a trigger/waypoint/script etc. You can use it as a switch to turn a trigger on and off.
However, in this case I don't think you need one. You do need to read Armsty's switch trigger tutorial. Your waypoints should look something like this
MOVE
MOVE
CYCLE <-------- synchro the trigger with this one
SEEK & DESTROY
CYCLE
The trigger looks something like this
Area: as you like
Activation box: East detected by West (or whatever)
Type: switch
Now, if you wanted this trigger to fire only if, for example, the attack group was moving in from a certain direction, you would need the variable. You'd set up another trigger in the approach looking like this
Activation: East present
On activation: Ullabella=true
And your first trigger, the switch one, would be the same except you would add a condition line
Condition: this and Ullabella