What does a phrase like "dit St Pierre" mean, when it follows a surname?
For example: Marie-Anne Romure dit St Pierre
In this case dit means alias. So Marie-Anne Romure was her given name but everyone knows her as Marie-Anne St Pierre and that is the name that she uses in every day life. There is a bit more information on this site that may help.
The French dit identifier is identifying something or referring to something about the person or person's family. It is not a conveyance such as de or du. It is not an alias, nickname or last name which are incorrect English translations!
French "dit" or "dite" names were''' identifiers''' used by French families, that describe the person based on a variety of possible definitions including French surname linage, maiden name genealogy, geography, guilds, master craftsmanship, military loyalties, seigneur relationships, military status ...etc etc etc
They are not "last names", "nick names" or "aliases." Unfortunately, English transcribers have incorrectly defined them as "aliases." The Old World French never referred to dits as "last names", "nick names" or "aliases." Originally dit did not have a true English translation and thus modern attempts have unfortunately followed English forms.
Many French Nouvelle Francaise and Louisanne families carried their dit identification from their origins in France, Netherlands and Britain when they migrated to North America.
Eventually most dit and dite identifiers use were dropped as families became nationalized