The Front-End Limit
As discussed on the Grate Limit page, the grate limit occurs when any increase in the rate of fuel delivery produces no increase in evaporation. In other words it represents the maximum rate of heat emission that a firebox can deliver beyond which point any additional fuel added to the firebox produces no additional steam.
The Front End Limit is a draughting limitation. In his paper titled "Two Point Four Pounds per Ton and The Railway Revolution", Doug Landau defines the Front End Limit as occurring when the available excess air falls below about 20% [i.e. when] complete combustion can no longer be achieved. If this occurs prematurely, the locomotive concerned would be deemed a 'poor steamer'. It could also be set by the designer at a value that would provide adequate steam, while at the same time avoiding 'uneconomic' combustion rates. The BR Standard locomotives were designed on this basis.
[In a letter to Chris Newman dated 10 Sep 2013, Dave Wardale wrote:  The definition given by Doug Landau is correct, although it should be added that the cause of excess air falling to a useable limit is due to the blast pipe and chimney characteristic.  Although BR claimed to have designed for this, Porta pointed out that whether the BR front end limit was by design or because they couldn't do any better was an open question.]
In the same paper, Doug goes on to describe what he calls the "Discharge Limit" which he differentiates from the Front Edn Limit as follows:
Discharge Limit: This is also sometimes described as the 'Front End Limit', but it is quite different to the condition described above. It occurs when the steam exhaust velocity reaches the speed of sound. At this point theory has it that the pressure/draught relationship breaks down. Curiously however, there are recorded instances of this limit being exceeded without apparent distress. It does however involve very high back pressures upwards of 14 lbs/sq.in., and was definitely something best avoided.