Compagnie de Duplessis means the "Company of Duplessis". Here is some background: François Duplessis (1689-1762) was a Captain in the Colonial Regular Troops (to read more about him please go to http://www.biographi.ca/009004-119.01-e.php?&id_nbr=1470&in...)
The first Catudal to come to Canada, Jean-Baptiste Catudal dit St-Jean came to New France sometime in the fall of 1721 or in the spring of 1722. There exists, to my knowledge, no ships record for him.
He was recruited in France at about the age of 16 and would have been under contract to serve a minimum of 6 years. Once recruited he would have first gone to either the fortress of Île de Ré which was the assembly point for most of the colonial troops or he would have been shipped farther south to Île d’Oléron for training before departing to New France.
Where he was stationed when he came to New France is not known but at the time of his marriage to Angelique in 1728 he was a part of the Company of François Duplessis. Although the marriage took place at Fort Chambly and Chambly became the town of residence of Catudal I think that he actually was serving with Duplessis at Baie-des-Puants which is in Green Bay, Wisconsin. That is where Duplessis on April 1727 received his full captaincy.
Duplessis also commanded troops in Fort Saint-Frédéic which is in Crown Point, New York. There was a time when Duplessis tried to transfer to Louisiana but was denied. The reason it is so important to follow the tracks of Duplessis is that somewhere out there are documents related to his movements and perhaps information on some of those who served under him. In this way we may be able to find out more details about our Catudal's origins.
So you asked where did those serving under Duplessis go, I think they were dispatched where ever they were needed; where ever the Indians or the English were attacking. Most people don't realize how large a land mass New France was - I've attached a map. New France covered, during its peak, the area from Newfoundland to Lake Superior and north from Hudson Bay to the Gulf of Mexico. Our Catudal would have likely not gone all over this whole area serving his King and country but he probably didn't stay put at the fort of Chambly either.
You asked what the troops did? Their primary job was to protect the interests of the Crown which meant they were to protect the people from the near constant attacks by the Iroquois - the friends of the English and the sworn enemies of the French. Did you know, during the early days during the colonization of New France that you had a 7% chance of dying at the hands of the Indians? The King had to send troops to New France to protect the people and thereby the fur trade otherwise no one would have wanted to immigrate to New France. It was not a popular destination for the people of France, what with the heat in the summer, the deep cold of the winters, and on top of all that, the attacks by the Indians.
It seems that the photo of New France that I had tried to attached to my reply to your questions Pierre did not upload or at least I can't see it, so I'll try to attach it here...