In current usage, a Shire is a territorial branch of the SCA. It is a sub group of the kingdom but is not part of a Barony nor does it have subsidiary groups attached to it. A shire reports directly to the principality or kingdom.
In ancient usage, however, this was not always the case. There are still extant Shires, founded prior to the introduction of this convention, that are parts of Baronies.
The Barony of the Middle Marches, for example, includes several Shires (Shire of Tirnewydd, Shire of Mugmort, Shire of Dernehealde), and new groups founded within the Barony who chose to be a part of it still retain the privilege of styling themselves as either Marches or Shires.