BEopt calculates internal gains from plug loads, major appliances, lighting, water heater tank losses, etc., all of which affect the heating/cooling load calculations. When BEopt is told how many occupants there are, all of these individual items are adjusted accordingly. (When BEopt is not told how many occupants there are, say for new construction, it uses the number of bedrooms as a proxy for the number of occupants.)

You can modify any of the individual items that contribute to internal gains, though the miscellaneous "Plug Loads" category is one of the biggest knobs available to you.