I'm pretty sure it works like this:
Waive with automatic revoke is to be able to trade a player after the trade deadline. In real-life MLB, it would be used to trade someone in August, for example. If someone claims him, you automatically revoke the waivers to keep him and the claiming team is the only team the player can be traded to.
Designate with revokable waivers is DFAing a player unless someone claims him on waivers, in which case you automatically pull him back. DFAing will remove him from your 40-man, but you can keep him once he has cleared waivers. Just make sure to assign him to a team when you get the notice he has cleared waivers. If you do nothing, he will be available to everyone else after 10 real days (not HBD days).