Hey Mathias,
thanks for your help. I will try to create an adjustment that reflects my understanding of the rules.
I looked up the german rules text and it states the same. However, I think this is not the intended meaning. I think I read somewhere that you get the opportunity to ask CGL for clarifications on rules from time to time. I would strongly suggest to ask for a clarification on this, because:
The way the rules are written now there is a clear intention visible to handle TMs and Spellcaster in the same way. Why not in sustaining there respective "Spells"?
The way the rule is written right now, sustaining spells would apply its modifier to damage soak rolls. Here as well is a clear intention that those rolls are never modified (save the opportunity to modify the relevant values for the dice pool like AP and Armor). There is even a logic behind it. All resistance rolls are "passive". No one does anything, so distraction of a sustained whatever will affect your defense roll (Rea+Int) but not your damage resistance, toxin or pathogen resistance or whatever.
That is why I thought sustain mods are equal to wound modifiers.
(I am still not sure how composure, judge intentions or other tests of that category should be treated.)
Just to clarify: I am totally ok with you taking all the rules absolutely literally. In the end, you are responsible product and you have to be able to justify your decisions and the only way to do so is waving (or throwing :-D ) the core rule book. But I don't think it makes sense this way and CGL should say something ^^.
So far, I will house rule it and try to implement an adjustment. Thanks for all the modifier names ;-)
PS: Reference to reddit, where the discussion arose as well:
https://m.reddit.com/r/Shadowrun/comments/41cdzv/sustain_a_spell_and_damage_resistance_test/