Answer

The principle of sufficient reason can apply to simple entities as well as composite ones. Even with simple entities, one can ask what the reason is for their existence, since nothing simply comes into being for no reason. If you know that there are mechanisms that produce entities, then there is no question about the reason for their existence, and only if they are composite do you ask how this particular one came into being. But that is a question of causality. In my opinion, the principle of sufficient reason also deals with entities that always existed, not only with those that were created. If they are distinctive, the question arises why they are as they are.
Of course, if there is a mechanism that produces many entities, and each time something different, that can serve as a reason. But then we have to ask what the reason is for that mechanism. This chain has to stop at a link that does not require a reason outside itself, one whose property is essential and necessary to itself.