The client can open a URL as either a full size window (if that is the frame used by the
client), or an overlay (such as a pop-up). The implementation depends on the client platform
capabilities, and the value selected may be ignored if the client does not support it. FULL_SIZE is supported by all clients.
Using OnClose may cause OpenAs to be ignored; if the client platform cannot
support both selected values together, OnClose takes precedence.
