Grammar of the Latin Language
Blanchard and Lea.
Derivation of substantives
4. substantives with the termination us (fourth declension) are likewise formed by changing the supine ending um into us. Their meaning is very nearly the same as that of substantives in io, and in some cases the same verb admits the formation of substantives both in io and in us -- as contemptio, contemptus; concursio, concursus; conscnsio, consensus; motio, motus; potio, potus. In some words of this kind in io, the abstract idea of what is implied in the verb is lost – as in legio, a legion; coenatio, a dining-room; region, a district.
Note. In regard to use, these forms are almost entirely arbitrary, one writer preferring the one, and another the other, without there appearing to be any difference in meaning. In some, however, there is a difference—as auditio, the act of hearing; and auditus, the power or faculty of hearing. As a third class of verbal substantives with the same meaning, we may mention those in ura, formed likewise from the supine -- as conjectura, pictura, cultura, mercatura. There are also a few verbs from which all the three kinds of substantives may be formed — as positio, positus, and positura (from pono); censio, census, and censura (from censeo). In some words, the ending ela, attached either to the stem of the verb or to that of the supine, conveys the same meaning as the endings io, us, or um — as querela (from queror); corruptela (from corrumpo, coruptum). Very nearly the same meaning is conveyed by some substantives ending in ium, in which the ium is suffixed to the stem of the verb—as judicium (from judico), odium (from odi , gaudium (from gaudeo), studium. (from studeo), refugium (from refugio, colloquium (from colloquor).