availability
Gets an object that can be used to determine availability of terrain from this provider, such as at points and in rectangles. This property may be undefined if availability information is not available. Note that this reflects tiles that are known to be available currently. Additional tiles may be discovered to be available in the future, e.g. if availability information exists deeper in the tree rather than it all being discoverable at the root. However, a tile that is available now will not become unavailable in the future.