46 CFR 69.59: Enclosed spaces.

Enclosed space means a space which is bounded by the vessel's hull, by fixed or portable partitions or bulkheads, or by decks or coverings other than permanent or movable awnings. No break in a deck, nor any opening in the vessel's hull, in a deck or in a covering of a space, or in the partitions or bulkheads of a space, nor the absence of a partition or bulkhead precludes the space from being included in the enclosed space. Portable enclosed spaces, regardless of method of attachment to the vessel, are treated as enclosed spaces as defined in this paragraph.