AC is a bit scattered in how it uses 'Signature' as an adjective, but the page about the concierge service appears to tie eligibility to the hard project cabin you are booked to travel in, and the Signature Class cabin is available on some domestic flights. ("Customers travelling in our Air Canada Signature Class cabin...[and] originally booked and ticketed in the following classes are eligible for select Concierge services: J, C, D, Z and P.) If you are booked in what it calls the 'classic Business Class cabin' you are not (at least according to that page).