The reason for having that extra level of abstraction is because there’s a many-to-one mapping between objects in the score and the playback effects they generate. The word ‘open’ and the ‘o’ symbol above a high hat are different Playing Technique events but they correspond to the same Playback Technique. Similarly, ‘mute’, ‘con sord’ and ‘avec sourd’ are different Playing Techniques mapped to the same Playback Technique. You can define your own, which could be text, symbols, or graphics. The Expression Map deals only with Playback Techniques, and you wouldn’t want to have to define separate entries for muted, con sord and avec sourd, as in sound terms they’re the same thing.
Internally we originally called Playing Techniques, ‘Playing Technique Appearances’, which may be a helpful distinction.
Re: sharing of custom playing techniques and playback techniques: you can do this by saving an Endpoint Configuration and using it in a Playback Template. If you have an expression map that references custom Playback Techniques, and there are custom Playing Techniques that use them, then those will all be saved in the Endpoint Configuration.