Sorry if this has been discussed before, but the search engine is down.

What is the different between a B fare and a Y fare ? I don't see the difference since the B seems to be unrestricted...

The reason I ask is that AF offered my company a one time upgrade from any Y ticket to a D class business ticket (whatever D is....).... and all the flights I'm looking at show up as B fare.

Thanks for the insight.

Cheers,

JB