What is the meaning of `struct X typedef` vs. `typedef struct X`?

The fact that both typedef <type> <alias> and <type> typedef <alias> are valid simply comes from the language grammar definition.

typedef is classified as a storage-class specfifier (just like static, auto), and the type itself is known as the type-specifier. From the syntax definitions in section 6.7 of the standard, you'll see that these are free to be interchanged:

declaration:    declaration-specifiers init-declarator-list ;declaration-specifiers:    storage-class-specifier declaration-specifiers    type-specifier declaration-specifiers    type-qualifier declaration-specifiers    function-specifier declaration-specifiersinit-declarator-list:    init-declarator    init-declarator-list , init-declaratorinit-declarator:    declarator    declarator = initializer

(Note, of course, that this is equally true for structs and for non-structs, meaning that double typedef trouble; is also valid.)

As others said, typedef is a storage-class specifier and as with other storage-class specifiers you are also allowed to put the specifier between the type and the declarator.

While this is valid and it is also a form that should be avoided as C marked it as an obsolescent feature:

(C11, 6.11.5p1) "The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature."

Both have the same meaning. Both of these two forms are valid:

typedef <existing_type> <new_type><existing_type> typedef <new_type>   

You can typedef the above struct in either ways:

struct X {    USHORT x;}typedef X, *PX;     // <existing_type> typedef <new_type> 

or

typedef struct {    USHORT x;} X, *PX;            // typedef <existing_type> <new_type>