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With =- hour, a new consistency check is required because f cannot be guaranteed to have been consistent half an hour ago at 9.30. If it has not been updated on the server (by another client) since 9.00 then it (the cached file) can be used, otherwise a new copy is required (either way and can be set to 10.00). If obtaining a new copy is necessary and that fails then the open fails.
Using 0 the client is optimistic. The more negative the the higher the level of optimism.