I know that there is an I-SID tag in the SPB MAC-in-MAC version for service isolation to implementing a VPN-like service. Also, it seems to me that B-VLAN is there to enable the SPB network to have Equal-Cost Trees (ECT) for load balancing between BEBs.
Couldn't these two functions (isolation and load balancing) or other functions be accomplished by just one tag (I-SID) instead of two tags (I-SID and B-VLAN)? There could be 16 million unique I-SID in a SPM network and seems enough to not to add another tag (B-VLAN) to the frame. Afaik, The I-SID is not hierarchical (like Q-in-Q) in order to stop service space waste... and it is weird to another tag (B-VLAN) to have a hierarchical-like model.
For further reading, refer to this link: https://www.al-enterprise.com/-/media/assets/internet/documents/spb-architecture-tech-brief-en.pdf