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Officially the service line is only in play for each serve. However I use it unofficially too as a mark to hit the ball back to a length from the rear corner. By hitting your drive above above the service line the ball will likely bounce behind the service box, taking your opponent into the rear court and giving you time to get to the 'T'.
One thing is for sure, if you hit your drive below the service line it will be generally be short of a good length! So next time you are struggling to get a consistent length out of the back corners aim above the service line and see the difference it makes!