A good rest can be defined as a rest where you can recover very well, 90 to 100%. For example, it can be sitting on a ledge, or a very good kneebar where you can stay for a long period, or a big jug with good footholds... It really depends on the climber feeling. It is also linked with how hard was the section before. If the section you are coming from was quite easy, you will not be so pumped and you will recover better than if you arrive totally pumped. A bad rest is just a short shake out. So a medium rest will be in between. You cannot recover very well but you can stay enough to calm down. A no hand rest is not necessarily a good rest. For example, a really friction dependant kneebar where the body core is at full load can be very demanding and you will probably feel more a medium or a bad rest than a good one even if you are able to release both hands. It’s all about feelings and there is no predefined answer depending on the holds configuration.