aberrant vs errant

Aberrant seems a subset of the word errant.

1. Thus, what's the effect of the Latin prefix 'ab-'? What are the similarities and differences?

2. What's this phenomenon called, in which a prefix or suffix affects nothing?

3. What are some more accurate words that characterise two words, X and Y, for which all of X's defnitions are contained in Y's, than 'subset'?

You have already noted the formation of the words and the significance of the prefix ab-, so where's the issue now?

err: to stray, OFr: errer

ab-: away; from

Errancy refers to a system's nature of deviation. Aberration is the instance of deviation. Errant behavior should be treated by behavioral correction, an aberration needs to be remedied by restoring to normal.

Also, the ab- prefix is generally not used with a positive connotation, while error by itself has no such baggage: "To err on the safe side."

HTH.