@Joe Riel Of course I agree the help page is confusing. At one point, it says that all entries of an rtable are "scanned" (as opposed to all opererands), and later that for sparse rtables only the nonzero ones are "scanned", so they can't both be correct.
But I expect (1) any mathematical operation on an rtable to give the same result, whether sparse of not, and (2) some algorithmic efficiencies may be be applied if it is sparse.
So I think the first point (and the first usage of scan in terms of all entries) means I expect to see seq return the number of elements of the rectangular size. In the case of mul (which has the same strange wording about "scan"), mul(V) returns 0, (as I expect) - it involves the same result as if non sparse, and must involve more that just the stored entries.
I took the second usage of scan (that for sparse rtables only the non-zero entries are scanned), to be associated with the second point, that somehow seq uses a more efficient algorithm, and can be faster because it doesn't attempt to access the non-zero entries.
But perhaps this is just me being optimistic that there is some resonable meaning behind the second usage (that @vv pointed to as strange)...