PCED N. 539/99 said:
3. The situation of the faithful attending chapels of the Society of St. Pius X is more complicated. They may attend Mass there primarily because of an attraction to the earlier form of the Roman Rite in which case they incur no penalty. The difficulty is that the longer they frequent these chapels, the more likely it is that they will slowly imbibe the schismatic mentality which stands in judgement of the Church and
refuses submission to the Roman Pontiff and communion with the members of the Church subject to him. If that becomes the case, then it would seem that they adhere to the schism and are consequently excommunicated. For these reasons this Pontifical Commission cannot encourage you to frequent the chapel of the Society of St. Pius X. On the other hand it would seem that you are among those who attend Mass in chapels
of the Society of St. Pius X because of the reverence and devotion which they find there, because of their attraction to the traditional Latin Mass and not because they refuse submission to the Roman Pontiff or reject communion with the members of the Church subject to him. At the same time it must be admitted that – this is an irregular situation, even if the circumstances which have caused it have come about through no fault of your own, and it should be remedied as soon as circumstances permit.