Let's say you need to configure Private VLAN on Cisco Nexus switch with MSTP. You create new VLAN, make it isolated and map it to primary VLAN. Your newly created VLAN is automatically mapped to MST0 and if your primary happened to be in any other MST instance there is a possibility that your primary and secondary VLANs ended up with different L2 paths. As a bonus, you get
annoying message "These secondary vlans are not mapped to the same instance as their primary" every time you run "show spanning-tree configuration". Not a big deal, but things like these irritate me. Not to worry, "private-vlan syncronize" under "spanning-tree mst configuration" will automatically map all secondary VLANs to the same MST instance as primary VLAN. The moment it's done, you MST digest changes and boom, you have brand new MST region and STP convergence on top of it. So, either pick exiting VLAN mapped to the same instance and convert to secondary community or isolated VLAN, or have all your VLANs mapped to MST0 and use other methods to load-share traffic between transit links.
I know what you are thinking. No, I did it in the lab, so you won't have to.