The idea there is that when you have an articulation, say, it might add one specific technique without removing all the others. So say you’ve got a technique like “bell” to hit the bell of the cymbal, and then when you add an accent, you can get an extra technique like “ping” (this is all made up nonsense, you understand), so if you have it set to ‘Add’, Dorico will then look for “bell+ping”, whereas if you have it set to ‘Replace’, Dorico will look only for ‘ping’, i.e. ‘bell’ will be removed.