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Extra threads say, "If an additional thread indicates “Rank” as a modifier, the magician can have each additional thread provide a bonus up to his Spellcasting rank".

Under Monstrous Mantle it says "Extra Threads: Additional Target (+Rank)".

Is this saying that a single additional thread allows up to Rank additional targets? o_O

Yes.

Most of the beneficial spells add +Rank targets, basically shifting a single target buff into a group buff. This saves having to waste time casting it every round (or other round) in order to give the benefit to everybody, and the durations of these spells have typically been extended a little bit to make them worthwhile.

Morgan (Panda) did a lot of work on the spell design, and I'm really happy with the way it came out. If you haven't read his recent blog posts talking about the Nethermancer and Wizard, you really should. It gives some good insight into the thinking behind why different things were changed, added, or removed.

Also "Extra Threads: Additional Target (+Rank)", extra threads that you can Weave are limited to your Circle extra thread limit. Circle 1-4 1 extra thread, 5-8 up to 2 extra threads and so on.

So at 5 circle you need to weave all spell threads plus you can weave up to 2 of those special effects extra threads.