What is it?
During this service, brew water pressure and temperature and steam pressure are set to optimal values using sophisticated test equipment. If purchasing a new machine, your machine will be unboxed, tested, adjusted as required and reboxed before delivery to you.
The "with modification" part of this service includes modifying the heat exchanger to set the brew water temperature to an optimal value while also setting steam pressure (usually higher than the default).
Why do it?
For great tasting espresso, brew water temperature and pressure need to be set to certain values within a small tolerance. If the brew water is too cold, sour, under extracted shots can result. If the brew water is too hot, the espresso can taste burnt and bitter. If steam pressure is too low, getting silky textured milk will be difficult or impossible. Getting a calibration done ensures your machine is performing the way it was intended (or even better ...)