Nothing more was said on it, as clearly Rockstar don't want to discuss this right now. It isn't clear whether we'll only be able to use the boot of a car at certain points in the game or on certain missions, or whether we'll be able to make use of it whenever we wish to.
"We also noted how, when Niko approached a parked sedan from behind in preparation to carjack it, an interface prompt approached in the upper-left corner of the screen: "Press LB to open boot." Boot, of course, is the British term for "trunk," leading us to wonder just what we might be able to stash in the backs of our cars-weapons, bombs, kidnapped people? Or even more interestingly, what we might discover in other people's boo-er trunks? For the moment Rockstar isn't saying."
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Source: GTAForums post