Hash tables are used to store reference data and from the practical point of view can be used as dictionaries. Also you can use this entity to organize the collection of unique records by defining unique shared key for a records and creating dictionary of already collected records.
The values of hashes can not be used in the data substitution operations, but you can write and read the dictionary elements by special commands, which works with the register. Hashes are context-independent, so all the data you have in the hash will remain unchanged throughout the entire time of digger run, unless, of course, you do not change them yourself.
For more details on using hashes, please see the appropriate section of the documentation.