What is an outside tank system?

In the Application Section of the Registration of Storage Tank Systems for Petroleum Products and Allied Petroleum Products on Federal Lands Regulations, point 2 says that "These Regulations apply to owners of outside, aboveground storage tank systems ...". In the Interpretation Section, no definition of "outside" is given.

If no definition is given in the definition or interpretation section of a regulation, guideline, or code, then the generally accepted meaning for the word becomes the legal definition. In this case, the intent of outside is meant to be out of doors or outside of buildings.

Tank systems that are housed inside buildings fall under the jurisdiction of the National Fire Code (NFC).

Is it an outside tank system if the tank was originally installed out of doors but has subsequently been enclosed?

Particularly for tanks that sit in open dikes, some departments have added a shelter to prevent rainwater from entering the containment space. In some cases, the shelter is attached to a building. These arrangements would still be considered outside storage tank systems.

Are tanks installed in structures such as vaults, sheds and quonset huts considered inside or outside tanks?

Tanks housed in structures such as vaults, sheds and quonset huts are considered outside tanks.

Is there a rule-of-thumb for determining if a tank system is an outside tank system other than those obviously installed out of doors?

The NFC definition of building is "any structure used or intended for supporting, or sheltering any use or occupancy". It might help to think of a building as a permanent structure built on a slab or foundation that includes a permanent egress. Tank systems, other than those installed in buildings, are outside tank systems.