We characterise the embedding of the spatial product of two Arveson systems
into their tensor product using the random set technique. An important
implication is that the spatial tensor product does not depend on the choice of
the reference units, i.e. it is an intrinsic construction. There is a
continuous range of examples coming from the zero sets of Bessel processes
where the two products do not coincide. The lattice of all subsystems of the
tensor product is analised in different cases. As a by-product, the Arveson
systems coming from Bessel zeros prove to be primitive in the sense of
\cite{JMP11a}