Visitors ask: What does a commemoration under "one conclusion" indicate?
If you check the the online Ordo and then examine "Liturgical Notes of the Society's Ordo for 2012," you'll find the following:
Commemorations are either privileged or ordinary.
Privileged commemorations are made in all Masses, and at Lauds and Vespers; ordinary commemorations are made only in Low Masses (and Conventual Masses) and at Lauds (but not Vespers). The Privileged commemorations are:
of a Sunday,
of a day of 1st Class,
of a day within the Octave of Christmas,
of the September Ember Days,
of the Ferias of Advent, Lent and Passiontide,
of the Greater Litanies (in Mass, no commemoration made in the Office).
All other commemorations are ordinary.
There are in addition certain Collects (with Secrets and Postcommunions) which are ordered to be said under one conclusion with the Collect, Secret and Postcommunion of the Mass. These added prayers referred to in this Calendar as 'commemorations', though not strictly such, are made in all Masses and are reckoned as only one prayer with the prayers of the Mass. Examples of such added prayers are those for the Pope, the Bishop and the propagation of the Faith. No mention is made of them in the Office.
On days of 1st Class and in High or Sung Masses (non-conventual) only one commemoration, which must be privileged, is admitted.
On Sundays of 2nd Class only one commemoration is admitted, that of a 2nd Class Feast, which however is omitted if a privileged commemoration is to be made.
On other days of 2nd Class, unless occurring on a Sunday, only one commemoration again is admitted, but this may be either privileged or ordinary.
On days of 3rd or 4th Class two commemorations (and not more) are admitted.
On days of 4th Class where one commemoration only or no commemoration is to be made the celebrant may, at a Low Mass, add one Collect (with accompanying Secret and Postcommunion) at choice. This Collect etc. may be chosen from any Votive Mass, or Prayers for Special Intentions or from Masses or Prayers for the Dead given in the Missal. This Collect etc. is said in the last place after any commemoration that is to be said.
If the Collect A cunctis from the Orationes Diversae is chosen the Titular of the church, or any chief Patron or for an Order or Congregation the Founder or Titular may be named.
In computing the number of commemorations any Collect, Secret and Postcommunion that is to be added, under one conclusion, to the collect etc. of the Mass being said is not taken into account.
However, in all cases, the Mass of a Sunday excludes the commemoration of a Feast or Mystery of the Lord and vice versa. Hence, where a Feast of the Lord, either perpetually or accidentally for that year, is celebrated on a Sunday no commemoration of the impeded Sunday is made. The term Feast of the Lord includes the Feasts of the Most Holy Trinity and the Holy Family, together with the anniversary of the Dedication of a Cathedral or Church. Likewise the Mass of the BVM or of any Saint excludes the commemoration of the same BVM or Saint.
To see the specific Notes for the Ordo of the F.S.S.P., check item III. Commemorations on page 11 of the F.S.S.P. 2012 Liturgical Ordo.