To provide free space for all enclosed conductors [314.1]. Please note that this
is the statement of the concept. What "free space" is will be made clear in the
details of the requirement.
Table 314.16(A) provides metal box dimensions.
For nonmetallic boxes, use only those durably and legibly marked (by the
manufacturer) with their volume [314.16(A)(2)].
None. You don't have to make any allowance for these [314.16(B)]. You could if you wanted to, but from an engineering standpoint that is unlikely to be worth the effort.
You can make a single allowance for the volume of the largest one present in the