[Mobhack]

If you hover your mouse over a button, there is help text for it in displayed the info window (the one at the top of the screen) - in this case if you hover it over the Target button it says "Select this unit as a target" and if you hover it over the exit button it says "Exit without setting a target".

By pressing the T for TARGET key or the target button you have now made that selected enemy unit the target for the firing unit. So the range will be extended, if necessary to allow fire at it. That is what the explanatory text says the button does, after all.

So - use the escape key or that big red exit button on the T key dialogue if you want to exit without setting a target.

Cheers
Andy