The interpertation of the parameters is dependent on the value of ulEffect. This function is considered "window related" when ulEffect is LI_BD_WND_ATTR or LI_BD_WND_STAT.
When ulEffect is not window related then the value of hwnd must be NULL and the ulMask value is ignored (ie no mask is used, the value provided in ulData is used to set the information).
When ulEffect is window related, the function optionally sends a WM_SETBIDIATTR or WM_SETBIDISTAT message to the window procedure whose information is being set. The window procedure can set the bidi information by itself, and/or perform any additional processing that is required when its bidirectional information is set. When LIF_NO_SENDMSG flag is specified no window message is sent by the function. This can be used by the window procedure itself when it updates its own window-related bidirectional information (so that it does not receive a recursive message).
It is recommended that the LIF_NO_SENDMSG flag is not turned on when the function is called from outside the window procedure whose attributes are being set. Applications should generaly use the default (send the message to the window procedure), so that the bidi aware window procedures can take the appropriate actions when the request to change their bidi attributes is made.
There may be cases where the LIF_WND_REFRESH and/or the LIF_CHILD_INHERIT flags are ignored. This can happen when a window procedure processes any of the messages sent by this function (i.e when the LIF_NO_SENDMSG flag is not specified). In these cases, it is up to the window procedure code to determine whether a redraw operation is requiered and whether child windows should process the message or not.
It is recommended that the LIF_NO_SENDMSG flag is NOT turned on, when the code executes outside the window procedure whose attributes are set.
Applications should generally use the default (send the messege to the window procedure), so that Bidi-aware window procedures can take the appropriate actions when the request to change their Bidi attributes/status is made.