Casks (sometimes referred to as real ale kegs) initially require a soft spile peg while venting beer. The soft spile is designed to allow excess gas to escape.
Once you (the cellarman) determine that the process is complete, this hard spile is inserted to ensure that gas is no longer able to escape from the cask. Conditioning beer properly will require both soft and hard spiles to be used to vent the cask.
Soft Spile sold separately.