One can derive nouns from verbs by attaching -us or -io to the perfect participle stem. For example, movere gives rise to motus (fourth declension) and motio. The meanings of these derived words are quite close to each other. What is the difference between -us and -io? I am not looking for a comparison of motus and motio, but a broader view of the difference of these to suffixes. I understand that a general rule will probably have exceptions.
Here is my understanding of the difference (which I would like to have corrected or verified): The suffix -io produces a noun for the general phenomenon related to the verb, whereas -us gives a single instance of the phenomenon or action. For instance, motio means motion in general (the fact that things move, ability to move) where as motus means a more concrete single movement or gesture. This division is not strict, however, and both suffixes can mean both things, but this is the general difference in tone. (Do not take this paragraph for a fact. These are just my thoughts that lack justification. The descriptions of motio and motus are supposed to illustrate my general point, not be accurate translations of these words.)