Something I have to keep explaining often when I am talking 40k tactics is effective ranges. Its basically common sense, its what range is your unit effective at. Knowing this will help keep your list efficient, meaning less time just moving and more time being effective.
Look guys/gals, you only have 5 certain rounds of play and sometimes less if armies are held in reserves. Lets make sure we know the effect ranges of our units and keep them in the battle for as much of the game as possible.
Lets look at a typical transport with melta's. Rhino/chimera moves 12" deploys melta troops at almost 3". (2" and 11/16" or something like that). Then your effective melta shot is 6". So far we have an effective range of just under 21". That is this units death zone. It kills incoming heavy armor that comes into this range.
Land Raider..... Moves 12" deploys almost 3", and then assault 6". yes its the same.
Plasma Gun squads. move 6" shoot rapid fire 12" or single shot at 24"
Furioso Librarian dread. "wings of sanquinus 12" move, 6" assault=18"
Often times, its simply the range of shooting. Remember that a flamer template is 8" long
The fact of the matter is, know the effective ranges of your units, and what is coming at you. This might be common sense but I see alot of people just not paying attention to it.