Internal space
Internal space is a generic phrase to describe the space within an enclosed boundary (as opposed to a space outside of that boundary).
It might be used in association with any kind of contained space - or a container, where in might be more specifically referred to as internal volume. Effectively the height, by the width by the depth.
In terms of buildings internal space may be more likely referred to as interior space (as opposed to exterior space) with the boundary being the building envelope.
The measures associated with internal space might be floor space, useful or total useful floor area (UFA/ TUFA), Total rentable area, treated floor area (TFA) etc - for more information see Floor area. The space would then be that area multiplied by the internal height but it may be used as quite a generic term and the specifics of what is included in different contexts should be defined. For example an unheated garage or cellar in a house might be considered an internal space but is not part of the treated floor area, because these spaces are usually uninsulated, unheated or untreated.
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