mhwolf It allows for the same difference in temperature between water in the brew boiler of a dual boiler and the temp when it hits the coffee. The HX being PID controlled effectively acts like a hot water jacketed brew boiler.
The Machine allows you to modify the built in offset by +5/-5C for fine tuning to your specific situation….This whole scenario is active when the machine is one of the BC modes, I keep mine in BC2 all the time.
P.S. The original programming allowed fine tuning of +/- 3C, but I asked them to increase it. This was because the BC1 to BC3 modes increase in aggressiveness of return to brew temperature e.g. capacity to make coffees one after the other. This might mean tuning the setup slightly differently if you have a busier situation.
It is an excellent machine and currently my daily driver. It has the convenience of dual boiler temperature control, with good steam just a button push and 30 or so seconds away.