The full text of that can be found at
notitiae.ipsissima-verba.org , perhaps by doing a search at
google.com. It is also in the book “Documents on the Liturgy 1963-1979”, page 1177.
It is dealing with a situation where another liturgical day overlaps. “Thus, for example, it can happen that the on the evening of the Fourth Sunday of Advent, when this falls on December 24, there would be at the same time the evening Mass of the Sunday and of the Vigil of the Nativity of the Lord.”
So there may need to be a decision about which Mass to use, Fourth Sunday of Advent or Vigil of the Nativitiy.
But it is not dealing with the situation we are discussing. Something to authorise using the Day Pentecost Mass at the Vigil, instead of the Vigil Pentecost Mass.