Y are completely unrestricted and survive a no-show. Read the penalties section, it usually says none (or they don't even show penalties) and it is fully endorseable to any other airline as it tends to be priced at the maximum IATA fare so all the other Y fares are equal. Switching up to J or F is jsut a matter of paying the difference.

B have some restrictions - often are mostly or even fully refundable, their main allure is more flexibility without fees than discounted fares and the ability to upgrade to C using points (as you can with Y) for no co-pay and a lot lower points.

The other difference is that as long as there are unsold coach seats there are Y fares for those seats but the B will often be gone, and even where there are both you might see Y7 B2 so they know they are cheaper and restrict the quantity. Some B fares are available on the route but only connecting from/to another route so you will see B seats listed but no B fares available as they don't sell the B without an inbound/outbound connection. Example the B seats YYZ-DFW cannot be bought YYZ-DFW but can be bought as part of YYZ-DFW-SCL.