Origin position in Marlin

Good evening,

I have some troubles understanding the position of he origin in Marlin.
What is the difference between X_MIN_POS and MANUAL_X_HOME in configuration.h ?
Let's say I have an endstop in the negative X, and that my nozzle is 20mm away from the bed when it is against the endstop. Which one of this 2 parameters should I define to -20 in order to set the endstop position to -20 and thus have my bed at X=0 ?

Thank you for your help ;-)