What is the difference between Q-in-Q and Private VLAN?

Question

Computer networks

Good afternoon.
Describe the difference between Q-in-Q and Private VLAN, please.
Familiar with Private VLAN technology. And then I came across such a concept as Q-in-Q stacking VLAN.
I read it and didn't see any difference. Both technologies are used to create VLANs inside VLANs, but there is information that they differ in some way, but I don’t understand what exactly.
P.S. Cisco has such a Cisco SPS224G4 switch, which just supports Q-in-Q stacking VLAN,
I didn’t notice this in other models. While Private VLAN is available only in older models.

Answer 1

When using QinQ, another tag is added to an already tagged frame. Most often used in providers. For example, from client 1, all frames are additionally tagged with 100, from client 2 - 200. Thus, you can distinguish frames in the vlan, for example, 10, from the first (they will be tagged 10 , then 100) and the second (tagged 10, then 200) clients.
Private vlan is used when it is necessary to influence the 'visibility' of hosts within the vlan. I saw hosters a couple of times - for example, there is a /25 network and a corresponding vlan, each host (one host is one client server) must be allowed to see only the default gateway and possibly a service server for loading the OS. In this case, the ports to which the router and the service server are connected operate in promiscuous mode (in terms of private vlan, again), each client port - in isolated mode (as a member of an isolated vlan). Double tagging here, as I understand it, does not occur.
In short, I think we can say that QinQ is a multiplexing technology, Private vlan is a port-level filtering technology (i.e. we can specify which ports to exchange traffic and which not, despite the fact that all ports are in the same vlan).