Well, just the game between two sessions.
Let's say you set up a randomly Formula A weekend with two FP's from one and a half hour, a qualifying from an hour a, maybe a Warming Up from an half hour and a full length race. With a total of 7 to 8 hours of gaming it is in my eyes just a really bad thing when you can not save your game between two sessions to continue on a later moment.
PCars want to be the game that stimulate the real racing world, but in real life you don't have to drive all the sessions of a racing weekend all in once after each other, right?