[dcl.meaning.general] p5 says

Thus, (for each declarator) a declaration has the form

T D

where T is of the form attribute-specifier-seq _opt decl-specifier-seq and D is a declarator. Following is a recursive procedure for determining the type specified for the contained declarator-id by such a declaration.

However, not all declarations consist of only T and D, such as

int i = 0;  // #1
template<class T>
struct C{
   void fun() requires (sizeof(T)> 0);  // #2
};

The complete declaration at #1 comprises the initializer while the declaration at #2 comprises a requires-clause. It can arguably say the form T D cannot be used to designate(match) these declarations at all. The subsequent subclauses that describe the type of a declaration all base on the form T D. Should we give a normalization to any declaration in [dcl.meaning.general] p5 in order to make these subsequence subclauses apply to these declarations? That is

Thus, for a declaration D0, a declaration has the form

T D

is produced where T is of the form attribute-specifier-seq _opt decl-specifier-seq_opt from D0 and D is the declarator of D0. Following is a recursive procedure for determining the type specified for the contained declarator-id by such a declaration.

Hence, for any declaration, we can abstract a normalization form T D from it such that the subsequent descriptions base on "T D" can work for that declaration.