Draft Bess DCI (draft-ietf-bess-dci-evpn-overlay-05)

Hi All
I am reading draft bess concerning dci and I don't see at all the meaning of this section:

**3.6. EVPN-VXLAN Interconnect for EVPN-Overlay networks
Downstream assigned VNIs in the Interconnect network.
In this case, the GWs and PEs MUST use the proper RTs to
import/export the EVPN routes. Note that even if the VNI is
downstream assigned in the Interconnect network, and unlike
option B, it only identifies the pair and
not the pair. The VNI used inside
each DC MAY be the same as the Interconnect VNI. GWs SHOULD
support multiple VNI spaces per EVI (one per Interconnect
network they are connected to)."

GW and PE must use proper RT for import export = ok.
I don't see wat exactly means "Downstream assigned", this means that the VNI number is not decided by me ?
I also don't see the relation between and the fact that we are in option B
Option B : MPBGP peering between DCGW and PE for example
There is one label per EVI ok, but what is the point exactly ?
The support of multiple VNI spaces per EVI is systematic for me, I understand the concept but I don't see at all the interest here.

Does somebody read this draft already, if yes , thanks for any help

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