[NTG-context] \definefallbackfamily check and force keywords

A very simpleminded question, if I may. What do the keywords, check and force do in \definefallbackfamily? Am I right to think that check=yes, force=no will use the fallback font only when characters are missing from the mainfont and check=no, force=yes will use the fallback font whenever it contains a character found in the source file? Alan