Vatican website translation:
28. In liturgical celebrations each person, minister or layman, who has an office to perform, should do all of, but only, those parts which pertain to his office by the nature of the rite and the principles of liturgy.
28. In celebrationibus liturgicis quisque, sive minister sive fidelis, munere suo fungens, solum et totum id agat, quod ad ipsum ex rei natura et normis liturgicis pertinet.
Slavishly literal translation:
28. In liturgical celebrations whoever, whether minister or [one of the] faithful performing his own office/task, does only and totally that which pertains to that [office/task] from the nature of the thing and the liturgical norms.
Continuing the set of norms drawn from the hierarchical and communal nature of the liturgy, art. 28 argues for a kind of “truth” in the ministers enacting the liturgy (as it will argue later, e.g., for a kind of “truth” in the times at which the various hours of the Divine Office are celebrated). This concern seems to arise from the earlier articulated understanding of the liturgy consisting of signs; obscuring the signs of the liturgy diminishes its communicative potential.
For the Eucharistic liturgy to manifest its hierarchic and communal character, its optimal form would involve a bishop presiding (and preaching), surrounded by a college of presbyters (who might concelebrate the Eucharistic Prayer and assist in the distribution of communion) and a college of deacons (who might carry the evangeliary, lead litanies, assist in the preparation of the altar and oblations, address the assembly with monitiones, and oversee communion in consecrated wine), and an assembly of the faithful, including catechumens/elect and the baptized, and from the baptized some who read the appointed scriptures, others who carry a processional cross, incense and thurible, candles, or assist at the altar, some serve as music ministers of song and possible instrumental accompaniment, etc. When such an optimal form is not possible, documents such as the GIRM indicate who is to do what in order to preserve the principle of art. 28. For example, in the absence of a deacon, a presbyter might read the gospel and a reader might speak or the cantor sing the intercessions of the Universal Prayer.
What the article seems to forbid is the practice, common at the time of the Council, in which presbyters (priests) “dressed up” as deacons or subdeacons to minister in those roles at a missa solemnis.
I suspect that such an interpretation may be controversial among some readers of the Pray Tell. I look forward to our discussion.