In a chemical reaction the limiting reagent is the substance that first gets used up, thus ending the reaction. 

Eating disorders often like to use this principle too.  For example, if a person skips 1 exchange of grain from their breakfast (those of you familiar with the exchange system for meal plans know what I mean), the ED might tell them that it's now a rule to skip that exchange at breakfast, and eating the full amount prescribed on the meal plan would be "excess."  This is often how restricting progresses over time, one limiting reagent after another. 

The myth the ED tells is that once this level of "control" has been achieved it would be a sign of weakness to revert back.  However it actually takes a lot of strength on the part of a person's true self to take back the control from the ED into their own hands and choose to follow the meal plan, which is one of the most powerful recovery tools.

Don't let slips with ED behaviors act as a catalyst for relapse.  Get back on track by doing the next right thing for your recovery.