The Pontiff alludes here to the fact that in the political and socioeconomic regime prevalent before the French Revolution certain professions generally were not exercised by nobles, since these were deemed beneath nobility. Their exercise implied, at times, the loss of noble status. One example was the exercise of commerce, reserved in many places to the bourgeoisie and the common people. These restrictions gradually diminished during the nineteenth and twentieth centuries and have entirely disappeared today.
In this passage, Pius XII seems to have in mind that the disturbances resulting from the two world wars had economically ruined a significant number of noble families. Their members were thereby reduced to exercising secondary activities, inappropriate not only for the nobility but for the high and middle bourgeoisie as well. One could even speak of the proletarianization of certain nobles.
In view of such harsh realities, Pius XII encourages these families not to dissolve in a prosaic anonymity, but rather to practice their traditional virtues and act with vigor and fruitfulness, thus communicating a specifically noble note to any work they exercise either by choice or under the harsh sway of circumstances. In this way they will make the nobility understood and respected, even in the most painful situations.
Plinio Corrêa de Oliveira, Nobility and Analogous Traditional Elites in the Allocutions of Pius XII: A Theme Illuminating American Social History (York, Penn.: The American Society for the Defense of Tradition, Family, and Property, 1993), 42-43.