[Meaning] Erases the background before drawing the window. [Value] 
PRF_ERASEBKGND
P R F _ C H E C K V I S I B L E
P R F _ C H I L D R E N
P R F _ C L I E N T
P R F _ E R A S E B K G N D
P R F _ N O N C L I E N T
P R F _ O W N E D