The newly-expanded canon 604 (with the latest edition in bold) now reads:
§1. Similar to these forms of consecrated life is the order of virgins who, expressing the holy resolution of following Christ more closely, are consecrated to God by the diocesan bishop according to the approved liturgical rite, are mystically betrothed to Christ, the Son of God, and are dedicated to the service of the Church.
§2. In order to observe their own resolution more faithfully and to perform by mutual assistance service to the Church in harmony with their proper state, virgins can be associated together.
§3. The recognition and establishment of such associations at the diocesan level belongs to the diocesan bishop, within his own territory, at the national level it belongs to the episcopal conference, within its own territory.
What does this change?
Basically, nothing! All can. 604 §3 does is make explicit what was already implied.
In the section of the Code which discusses associations of the faithful in general, canon 312 details which authorities are able to formally recognize and approve such associations. Naturally, the Holy See (a.k.a. “the Vatican” or “Rome”) has competence to approve international associations; the relevant bishops’ conference has the competence for approving associations within its own territory; and the diocesan bishop is competent to approve diocesan associations of the faithful within his own diocese.
Even while there can be different kinds of associations specific to members of the Christian faithful in different states in life—such as clerical associations for priests, or associations specifically intended for laypeople—can. 312 was meant to apply to all associations of the faithful inclusively.
If this doesn’t really change anything, then why add it to the Code?
I don’t know for sure. The Pope didn’t call me to chat about this motu proprio beforehand!
But if I had to hazard a guess, my thought is that perhaps there were some questions or mistaken impressions that the Congregation for Institutes of Consecrated Life and Societies of Apostolic Life (CICLSAL) needed to approve all associations of consecrated virgins, not just the international ones.
The 1988 document Pastor Bonus, which describes which Vatican dicastery (or “department”) handles what, states in art. 110 that CICLSAL has competence for “the order of virgins and their associations.” So even while I would have always presumed that CIC can. 312 applied to associations of consecrated virgins just as it did to any other association of the faithful, I could theoretically imagine a scenario where someone might have wondered whether this specification in Pastor Bonus art. 110 meant that associations of consecrated virgins were somehow a special case reserved to Rome.
So now, it is definitively clarified that associations of consecrated virgins do indeed follow the same norms applying to associations of the faithful in general.
Interestingly, there seems to have been similar questions and concerns about can. 604 §2 back when the Code was initially being drafted. It was argued that since all Christians fundamentally have the right to associate, it could be unnecessary or superfluous for the language of the Code to go out of its way to specify that consecrated virgins enjoy a right common to all the faithful.** But at the end of the day, the drafters obviously decided to err on the side of clarity. And so can. 604 §2 exits to note that consecrated virgins can indeed form associations among themselves for pious reasons.
*For a good explanation of all the changes introduced in this motu proprio, see this article in The Pillar.
** Sr. Sharon Holland recounts this in her article “Consecrated Virgins for Today’s Church.”