The resonance/device rating is used in a defensive manner if some decker decides to throw a data spike at you.
You defend against Data Spike with Intuition + Firewall (which for a Technomancer is Willpower).
You use Resonance for six actions (Erase Resonance Signature, Compile-, Register- & Decompile Sprites + Thread- & Kill Complex Forms)
But yes, Matrix damage (which is caused by for example Data Spike) is resisted with Device rating (which for a Technomancer is Resonance) + Firewall (which for a Technomancer is Willpower). Fading damage is also resisted with Resonance + Willpower (while Biofeedback is not).
limit to how high a level you can thread a form.
True, but then again, the limit is
three times your Resonance rating. Even with a Resonance rating of 4 let you thread level 12(!) Complex forms. Due to fading values you are probably better off threading them at lower levels and if you need a lot of hits instead push the limit with Edge.
Personally I would try to keep the fading value down to a minimum. You can thread Cleaner at level 1, Static Veil at 3, Resonance Channel at 3, Stitches at 4, Transcendent Grid at 5 & Tattletale at 4; and still only have a fading of DV2. You can do all that with a Resonance Rating as low as 2.
Cha is more than just Attack, Its also the number of Registered sprites you can have.
You keep saying that, but it is only the number of Registered sprites you can buy for for a cost of 1 left over Karma per task in Step 7 during chargen (for reference see SR5 p. 98 Additional purchases & restrictions (table))
Yes, I know of The errata. The errata did nothing to change that. Only add in logic when you first start off.
I think that you maybe got the two mixed up...?
Charisma is used as a limit during chargen (SR5 p. 98)
Logic is used as a limit post chargen (SR5 p. 256 + errata p. 3).
SR5 p. 256 Registering a Sprite + Errata p. 3 REGISTERING SPRITE LIMIT
The Matrix doesn’t know what to make of sprites, so they show up as illegal activity. This puts a limit on the time you can keep a sprite around before GOD finds it and crashes it. You can increase your sprites’ longevity by registering them with the Matrix. This process takes a number of hours equal to the sprite’s Level; during this time, the sprite’s Overwatch Score does not increase due to time, and neither you nor the sprite can take other actions. At the end of this time, make an Opposed Registering + Resonance [Level] v. the sprite’s Level x 2. This causes Fading of 2 DV per hit (not net hit) the sprite gets, minimum 2 DV.
If you get at least one net hit, your sprite is registered with the Matrix. Its OS is erased, but can be restarted if the sprite performs an illegal action. Add your net hits on the Registering test to the number of tasks your sprite owes you. The sprite is now a registered sprite and no longer counts toward your limit of one compiled sprite at a time. Your registered sprite will stay with you as long as it still owes you at least one task. You can register a number of sprites equal to or less than your Logic attribute. Everything else is the same as for compiled sprites. Almost everything.