I don't know how it's done in your province in Canada, but here in the States there are specific terms like "Salvage" not "no brand unfit title (not salvage or irreparable). AFAIK there is no such thing as irreparable, though it might vary by state. If the title has a specific classification it might help to state what that is.
Yes undercarriage damage can mean the similar/same structural problem as if the frame was damaged, since you wrote car so unless it's very old it would be unibody so the undercarriage floorpan of the car is the equivalent of the frame. It could be cost prohibitive to fix something with significant damage or might just need trued and/or structural support welded on. In my US state a salvage vehicle can be repaired and inspected to become road worthy again.