xor     ebx, ebx                ; More effective than push 0
                mov     edx, IDI_APPLICATION
                invoke  LoadIcon, rbx, rdx
                push    rax                     ; hIconSm
                mov     r15, rax
                push    AppName                 ; lpszClassName
                push    rbx                     ; lpszMenuName
                push    COLOR_APPWORKSPACE + 1  ; hbrBackground
                mov     edx, IDC_ARROW
                invoke  LoadCursor, rbx, edx
                push    rax                     ; hCursor
                push    r15                     ; hIcon
                invoke  GetModuleHandle, rbx
                push    rax                     ; hInstance
                push    rbx                     ; cbWndExtra &  cbClsExtra
                push    MainWndProc             ; lpfnWndProc
                mov     eax, CS_HREDRAW + CS_VREDRAW
                shl     rax, 32
                mov      al, sizeof.WNDCLASSEX
                push    rax                     ; cbSize & style
                mov     ecx, esp                ; Establish pointer to WNDCLASSEX
                invoke  RegisterClassEx, rcx
                test    ax, ax
    

An atom. For more information, see About Atom Tables.
This type is declared in WinDef.h as follows:

Code:

typedef WORD ATOM;    

never test the return value of RegisterClass() or RegisterClassEx()
Since a class an only be registered once the second caller will fail if it tries to register the class again.

Instead of trying to detect this using GetClassInfo(), a solution that does not work in a multi-threaded program due to a race condition; it is better to simply ignore the return value from this function and pick up the failure when CreateWindow() is called as that will fail due to "class not registered".