is an object that convert-to-screen can recognize, typically a pane or simply
is not supplied or is
is a list of formats that can be loaded. All the formats in the list can be loaded, but on Cocoa and Windows the list is not exhaustive, and it may be possible to load formats that are not listed.
If for-writing-too is supplied as non-nil, then formats is a list of types that externalize-and-write-image can write. In this case the list is exhaustive on all platforms, and externalize-and-write-image can write a format if and only if it appears in the list.
CAPI User Guide and Reference Manual (Unix version) - 25 Feb 2015