What is the difference between combustion Eff and boiler Eff?

Combustion Eff, as calculated by an FGA, is a measure of how efficiently the fuel is being burnt.

Boiler Eff, as defined by the boiler manufacturer, also takes into account other factors that cannot be measured by an FGA. For example, the heat lost from the casing of the boiler or for a steam raising boiler its 'blow down' losses.