How can I tell if I have a variable speed HVAC or whether I need it?

I read a website that says, "Zoning a single-stage system is always going to be a sub-par design." They basically say a zoned HVAC system is inherently bad if it is single stage. That kind of scares me because I not only have a zoned system, I have a system where a zone was added. When the zone was added the HVAC company said that my system had the "capacity" to add a zone. We were basically dividing up an existing space and splitting it into two different zones, not adding new space. However, the website linked above says specifically that adding a zone to a single-stage system is an especially bad thing to do.

My system has 4 zones and 2 heat pumps and two outdoor condensers/exchangers. One heat pump has one zone and the other has 3 zones. It has a bypass damper on the return of the 3-zone heat pump.

How can I tell if I have a single stage system or a multistage system? If I have a single stage system, should I be analysing it to determine if I should convert it to a multi-stage system?