You mean ‘unnamed’ is what’s confusing you?

Normally you can do anonymous struct/union members or struct struct/union members that are tagged structs but not anonymous.

I.e. in standard C you’d have to do either:

struct foo { int baz; };
struct bar { struct foo foo; };
...
struct bar data;
data.foo.baz = 0;

or:

struct bar { struct {  int baz; } foo; };
...
struct bar data;
data.baz = 0;

but to do the following, you’d need the extension:

struct foo { int baz; };
struct bar { struct foo; };
...
struct bar data;
data.baz = 0;