As you say, an RCD has two ratings - the 10mA, 30mA, 100mA is the amount of earth leakage current that will trip it (measured in milliamps so is very small. 30mA is usual for equipment to be used out of doors; 10mA is very sensitive and used e.g. in hospitals where patients have wires and tubes poked into them. 100mA is not very sensitive so can detect certain faults including those can can start fires, but may not stop you getting a shock, though it is not usually fatal. The problem with having a very sensitive RCD is that it might trip from non-dangerous events and be a nuisance.
There is also a rating for the amount of time the fault current will persist before the circuit is broken. this is usually 30 or 40 milliseconds but can be deiberately delayed to give discrimination or reduce nuisance tripping. The makers of RCDs can give you a chart showing time/current graphs and the effect they can be expected to have on a person receiving a shock.
The other rating, 32A, 60A, 100A is the current-carrying capacity. So to protect a 32 amp ring main or garden sockets you would use a 30mA/32A RCD; but to protect an entire house you might use a 100mA/100A RCD (though this is not always considered a good idea).
If you tried to protect a 100A house installation with a 32A RCD it would be running above its capacity, overheating, contacts being too small and might arc or stick under excessive load.