Catholics are confused by these titles, as well as most non-Catholics.
The meaning of the prefix is along the lines of cooperator, as in the fact that she cooperated perfectly with God's plan, never refusing Him, beginning with her yes to being "overshadowed" by the Holy Spirit, as recorded in Scripture. In this sense, she is called co-redemptrix and co-mediatrix, since her cooperation with the Holy Spirit brought our Redeemer, and Mediator of all grace, into the world.
Since the title causes so much confusion, popes have been reluctant to add it to her formal titles.
That one word co-operator can be understood (misunderstood?) to suggest including an initiating of the proceedings on her own part as co-initiator sort of thing, while continuing that into perpetuity even though the into perpetuity aspect is rather assumed to be true, as it is also the portion which many find problematic in a theological sense, all the after-the-fact explanations for the central-most assumptions concerning it being not enough to establish that such considerations in regards to Mary having been inclusive of the Gospel as originally preached -- best as that can be reconstructed through both biblical and earliest patristic evidences, these latter needing be examined and sifted for opinion expressed in relative isolation before some of those isolated expressions, in regards to Mary in this instance, later converge over time.
In practice [within Catholicism] what would or could be considered the working of the Holy Spirit within a person is at times attributed to "Mary" when such workings of the Spirit are said to convey something of a feminine aspect in (spiritual?) nature.
There was a link provided on another thread a few days ago now, one which I had seen posted on these pages before -- but could not find it just now when searching through literally hundreds of comments.
The linked-to article (which I now cannot find) mentioned a man who (if memory serves) related while he was yet "Protestant" before later converting to [Roman] Catholicism had experienced the Holy Spirit minister to him tenderly, in such a way as the man sensed or regarded as being feminine.
Shortly after his conversion he related this to a Catholic priest and was told "that was Mary" rather than the Holy Spirit.
Would you know of the account I am speaking of? The [theological] trouble I have with that sort of identification is that Christ, when speaking of the Comforter being sent to us -- this "Comforter" had in the earliest beginnings of the Church been understood to be the Holy Spirit -- not "Mary" or some other identifiable entity.
If it be that now, Mary is fully united with Christ, then so be it. I would wish & hope that that be the case. Yet in that sort of thinking, how to not then to begin considering them all to be of one substance, my using here the term substance (or the alternative "essence") much in the same manner Athanasius pressed for recognition that the Father, Son, and the Holy Spirit were all of one substance.
And if of one substance, (now with Mary and countless saints?) then why refer to this substance by multiple names, again separating identities if it as as otherwise also said in apologetic that all the various God-like powers & abilities widely attributed to belonging to Mary, she derives from any selection/combination of the three Persons of the Trinity?
If saints (including Mary) all stay separate enough to remain addressable to we mere earthlings by name -- if those individuals are capable of producing miracles attributable to themselves by name -- how have we not made a return towards polytheism in effect and in truth (returning to a sort of ancestor worship, this one with One Big God (in 3 persons) and a bunch of jr. grade gods (Mary the highest ranking of these in the more modest appraisals) with this all being explained and excused (made excuse for) by numerous rhetorical limiting qualifiers which are forthwith abandoned just about as soon as the critics of these sort of proceedings are out-of-sight, no longer raising objections?
'One could kick them out of the RC Church I suppose -- without needed merely "suppose" for one instance of that is said to have happened in the case of the last person put to death by the Inquisition...
> "The last official Spanish execution for heresy was in 1826, when a schoolmaster was hanged for substituting “Praise be to God” for “Ave Maria” in school prayers. The limpieza de sangre statutes remained valid (though increasingly unenforceable) until 1865.History of Christianity Paul Johnson p. 308