Remaining Capacity is the difference between the Usage (or total work) and the selected Capacity Scenario. The selected Capacity Scenario can be designated as the Availability. You may have multiple Capacity Scenarios, but SELECTED one is that which you choose to use the heatmap for. Remaining Capacity Scenario will only display in the Resource Analyzer when you have created a Capacity Scenario and the selected view is Totals by Role.
View Total Column Options with No Capacity Scenario
Note the choices displayed under Available Columns when there is no existing Capacity Scenario created.
View Total Column Options with an Existing Capacity Scenario (New Hires 2013)
Note the choice of "Rem. Capacity" displayed under Available Columns when there is an existing Capacity Scenario (New Hires 2013) created.