Metabolic Rate is used to describe the amount of heat given off by people within a space. By selecting the activity most prevalent in the building you can model how the occupants of the project will impact total energy use. People who are more active will have a higher metabolic rate, and in turn give off more heat which must be addressed by the building HVAC system.
A common reference for activities and their corresponding heat gains is ASHRAE Handbook Fundamentals. Chapter 18, Table 1 is reproduced below providing activity levels and heat per person. The cove.tool options of sitting(60), standing (70), walking(200) and sports(500) are in order of low to high metabolic rate, and therefore heat gain.