Three classes with one rulebook, the LM GTE and GTLM situation is fairly simple. Manufacturers go out and build their preferred road car into a GT car based on rules laid down by the Automobile Club de l’Ouest. This should get the cars pretty close on performance but some balancing does go on, to keep the action close.
Cars available now include the Aston Martin Vantage GTE, the Ferrari 488 GTE and the Porsche 991 RSR with the new arrival, the Ford GT. These are the LM GTE cars and the difference between Pro and Am is all in the driver lineup. A Pro car can have, as you would expect given the name, a full crew of professional racing drivers. An Am car however must have at least one driver graded Bronze or Silver in the line up.
The ACO handles driver grading in association with the FIA.
GTLM is the same rulebook as for the LM GTE cars but they run in the States and in order to engage better with the audience, IMSA will grant waivers to teams to bend the rules and come racing. For example, the rules say the engine should be no larger in displacement than 4.5l, but both the Corvette and the SRT Viper have bigger powerplants. IMSA have said this is ok but use performance balancing to reduce the advantage it offers.