Second append for Caster Artoria, she has little use for the other two, while easier access to her NP is highly useful.
Second append for Charlie, that and his 30% battery would allow you to use a 50% CE like Traces for looping. That is how I use my Summer Okita Alter, and he works basically the same from what I've heard.
Don't know enough about the Trung sisters, but probably append 2 again.
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Append 1 is best on solo servants, like Heracles, where the boosted damage and extra effects are used every turn.
Append 2 is best for when you want to NP earlier. This is the majority of servants.
Append 3 varies.
On Berserkers where it just reduces the chance of critical hits from a single enemy class, it is kind of a waste, and should only be done with you are basically finished with the servant otherwise.
On other classes, it instead offers bonus damage against a single class (excluding Mash, who gains bonus damage against all Extra class servants), and the usefulness is still mostly low. But sometimes they will have a bonus against a class you would want to fight with them.
