This is the current limit for the LED.. Pretty basic. The vast majority of common LEDs have current limiteds of 20ma, with some bright LEDs handling 30ma without much trouble, beyond that it's up to the datasheet for your specific LED.
It has nothing to do with reverse conduction of a diode this is not taken into consideration in a resistor calculator typically.
If you want to be safe, assume this current is 10ma and run your calculations from there, your LED's will last many times longer than one run at it's fully rated current and will consume less power but still provide an appreciable amount of light. Considering the main goal of many LED setups is to simply be able to be seen (often in the dark) not necessarily as primary illumination extracting every lumen of brightness out of the diode isn't as high a goal as you might think. Given running a diode at 10ma vs 20 consumes half the current, it generates 1/4 the heat and more than doubles the LED life yet will produce more than 50% of it's rated light output at full current, so it's always more efficient to under current an LED from it's maximum specs.