Strictly speaking it is always ADS as the supply is automatically disconnected on a fault.This makes sense now as in TT system we have higher earth resistance so we cannot rely on ADS to occur hence we need time delay 100mA RCD for fault protection and then for higher risk areas 30mA RCD for additional protection.
The difference is on a TN system then usually you can achieve it on the over-current protection device (fuse, MCB, etc) due to low Zs if a reasonable design permits it, but on TT it almost always has to be an RCD due to the relatively high Ra (and so high Zs and low PFC).
But you can get situations on TN with high current final circuits or sub-mains where it is not feasible to use OCPD to meet the disconnections times and so then you are back to using an RCD in combination with OCPD. For many final circuits that can just be an RCBO, but for sub-mains you might be looking at fuse-switch or MCB + delay RCD combinations, or fancy (and expensive) MCCB that feature adjustable earth leak trip settings, etc.