It is an intention in which one will not let go of the continuous feeling of resentment and in which one retaliates measure for measure. Its function, not to tolerate, is easily understood.What is resentment? It is not letting go of an obsession which develops through association with the anger which underlies it. Its function is to be the basis of non-endurance.-Abhidharmasamuccaya
Regarding these two mental events, the Ratnamala states,
Indignation is an unsettling event in the mind,
And resentment comes in its wake.