Steady State Efficiency measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily. However, furnaces cycle on and off as they maintain their desired temperature. Furnaces typically do not operate as efficiently as they start up and cool down. As a result, steady state efficiency is not as reliable an indicator of the overall efficiency of your furnace as AFUE.
SSE is calculated by allowing a furnace to warm up, (typically for at least 20 minutes), then measuring the input BTUs per hour (BTUH) from the fuel and output BTUH released to the house in the form of heat. You then divide the output BTUH by the input BTUH.
For example, a furnace that outputs 62,250 BTUs per hour once it has warmed up, and requires 75,000 BTUH of fuel has a SSE of 62,250 / 75,000 = 83%.