If the player has more than one GPU and at least one of them is PhysX-capable, PhysX is set to normal by default. If the player has a single DX11-capable PhysX-capable GPU with at least 1 GB of video memory, PhysX is set to normal by default. In any other situation, PhysX is disabled by default.
If the player does not have a PhysX capable GPU, or the player has a single GPU but it has 512 MB or less of video memory, they will get a warning popup message when they try to enable PhysX normal or high. But anyone can ignore the warning and enable PhysX regardless of their hardware.