SubStack allows players to use skills from an X number of other classes. It's all about which subclass a player choses. The type of subclass a player can select can be bound by class tree, race or type (fighter or mystic). Substack is all about what subclass players chose to find the righ mix.
Multi Skill on the other hand is not bound by the requirement to even make a subclass. From level 1 players can learn skills from other class trees, races or types (fighter or mystic). With Multi Skill it's all about what skills players chose to find the right mix and fit their play style.
I see why both would seem similar, as both deviate from the normal L2 class system (Damage Dealer-tank-healer/buffer). It's all about the skill delivery system, and in that respect substack and multiskill, are very different.
I just think that it would be great if I, and others, could properly label our servers. Just a suggestion.