I've been looking for this and everywhere and despite the multiple guides on the Internet and on this forum describing the rules for the branded fare class on Air Canada, one thing I could not get is how to force a booking in a specific fare code. This has a direct impact on the number of eUpgrade credits required so it is certainly useful. For example, M class in FLEX generally requires only 1-2 credit.

On United website, it is possible to search and book a specific booking class. I haven't found the same thing on Air Canada. It is possible to call the reservation line to ask for a specific booking class but knowing the price ahead of time can be useful. I just realized that it you can search for those fares in ITA Matrix.

The trick is to select "Show Advanced Controls". And then use the following two options:

• Routing Code: O:AC (to force Air Canada)
• Extension Code: F ..m-fl

The extension code is where the magic happens. "F .." is to search a fare basis. The Air Canada fare basis rules will start with the fare code and end with the branded fare. The "-" is a wildcard so "F ..m-fl" is to search a FLEX fare in M class. The other possible brand codes are the following:

• Economy basic: lgt
• Standard: tg
• Flex: fl
• Comfort: co
• Latitude: lt
• Premium economy lowest: pl
• Premium economy flexible: pf
• Business lowest: el
• Business flexible: ef

Using the above allows one to price a flight with different fare basis to see if it worth going from say a H class that could require up to 8 eUpgrade credits in Flex to a M class requiring a max of 2 credits.
Unfortunately if you want to book, Air Canada reservation or a travel agent will need to get involved but at least you can search for the pricing in advance.