The value is an NSNumber containing a BOOL, which affects the interpretation of Proportional scaling types. A NO value will make the image fully visible, but there may be empty space on the sides or top and bottom. A YES value will cause the image to fill the entire screen, but the image may be clipped. If this is not specified, NO is assumed. Non-proportional scaling types ignore this value.
Available in Mac OS X v10.6 and later.
Declaration Syntax Version Information- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later)