[dcl.init.aggr] p16 state

Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the elements of a subaggregate;

Presumably, the initializer-list refers to the initializer-list enclosed in the brace-list-initializer of the aggregate initialization. Consider this example:

struct A{
   int a;
};
struct B{
   int arr[2];
   A obj;
};
int main(){
   B b = {0,1,{2}}; // #1
   B b2 = {{0,1},2}; //#2
}

In this case, the initializer-list at #1 is 0,1,{2} that does not begin with a left brace.

If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clauses from the list are taken to initialize the elements of the subaggregate;

The initializer-list at #2 is {0,1}, 2 that does begin with a left brace, however, we want the brace elision rule can apply to the initializer-clause 2 when initializing the subaggregate B::obj. In such two cases, we just intend to mean whether the initializer-clause begins with a left brace or not.

Maybe, change [dcl.init.aggr] p16 to

Braces can be elided in an initializer-list as follows. If the initializer-clause in the initializer-list that would initialize the subaggregate element is not a braced-init-list, only enough initializer-clauses from the list are taken to initialize the elements of the subaggregate; the elements are considered to be explicitly initialized. Any remaining initializer-clauses(if any) are left to initialize the next element of the aggregate of which the current subaggregate is an element.

if the initializer-clause is a brace-init-list, I think it is not necessary to specify in this paragraph. That would be redundant since it is just a normal aggregate initialization.

The emphasized wording may be more precise if it is changed to

If the initializer-clause in the initializer-list that would initialize the subaggregate element is not a braced-init-list, only enough initializer-clauses from the list are taken with comma-separation to form an initializer-list D, the subaggregate is initialized by braced-init-list { D }