pszClassClient (PSZ) - input
If pszClassClient is not a zero-length string, a client window of style flStyleClient and class pszClassClient is created. pszClassClient is either an application specified name as defined by WinRegisterClass or the name of a preregistered WC_* class; see Preregistered class names are of the form "#nnnnn", where nnnnn is 1 through 5 digits corresponding to the value of the WC_* class name constant.
If pszClassClient is NULL, no client area is created.
This parameter can also be specified directly as a WC_* constant.