Dry nitrogen left in new piping or equipment to keep moisture out and reveal leaks.
A holding charge keeps positive pressure of dry gas in piping or equipment that will not be commissioned right away. It keeps humid air out and shows a leak if the pressure has dropped when you return. Tag it with the pressure, date and ambient temperature so the next person can judge a change correctly. The Red Seal task matrix lists "Applies HVAC/R holding charge" as sub-task D-11.08.
Exam work areas
D · Installing systems and controls