Confusion about the definition of "explicitly initialized element" in aggregate initialization

In Appertainment

Each initializer clause in a brace-enclosed initializer list is said to appertain to an element of the aggregate being initialized or to an element of one of its subaggregates.

So

2. the explicitly initialized elements of the aggregate are
   the elements with an appertained initializer clause
   and the elements having a subaggregate with an appertained initializer clause.

for your example

S2 y = {1, 2, 3, 4};
// 1 appertains to y.s.a 

1 doesn't appertain to element y.s, but one of its subaggregate (y.s.a as noted).
y.s is a *"element having a subaggregate with an appertained initializer clause".