Chris,
I think he means the use of a spill back tank, now not used, I have only seen a few in my 55 years in the trade, usually a large tank on the plant room floor 1/2 full of water, plastic balls floating on the water, and a spill back PRV when the water in the system expand it spills back into the tank, when the system cools down the pressure set puts the water back into the system.
Normally on a big job with a high head that won't support a membrane in an expansion vessel, the last one I worked on was about 10 floors offices, about 4.5 Bar system pressure with steel tube boiler rated at 6.5 bar.
PS Usually large volume of expansion required