I suppose that Damage Threshold covers some of those situations - U_K, assuming no immunity, I presume resistance first, then Damage Threshold?
Yes.
And where immunities exist, you can't have a resistance to the opposite?
As a rule of thumb I think that works well, I wouldn't call it a hard rule (such as in the case of Tiamat) but each Immunity should have one Vulnerability - it does not always need to be the opposite though.
So if space deals 3d6 fire and 3d6 cold each round (call it average 10 for simplicity, even though slightly higher):
Space deals: 18 Cold, 12 Radiant and 6 Necrotic each turn.
Entity A has fire immunity & cold vulnerability (+50%)
Entity B has fire absorbtion & cold super-vulnerability (+100%)
Both entities have Damage Threshold 10.
Entity A: 0 fire damage, 15 cold damage, less damage threshold 10 = 5 cold damage per round.
Entity B: 10 fire healing, 20 cold damage, less damage threshold 10 = 10 cold damage per round, offset by equal amount of healing.
No. Damage Threshold is not a Reduction. Just a flat number you overcome - much simpler this way.
If Godzilla has Damage Threshold 60 (or whatever) and you deal 60 damage the damage is 0. If you deal 61 damage, the damage is 61. You only exceed the Damage Threshold to do damage.
Space = 18 Cold, 12 Radiant & 6 Necrotic.
Entity A: Is Fire Immune and Cold Vulnerable with Damage Threshold 10
Takes 27 Cold (+50%), 12 radiant and 0 necrotic...overall 39
Entity B: Fire Absorber, Super Cold Vulnerable and Damage Threshold 10
Takes 36 Cold (+100%), 12 radiant and 0 necrotic...overall 48
Entity C: Radiant Absorber, Necrotic Super-Vulnerable (+100%) and Damage Threshold 10
Takes: 18 Cold, heals 12 from radiant, takes 12 from Necrotic...overall 18.
Entity D: Cold Absorber, Radiant Immune, Necrotic Vulnerable and Damage Threshold 10.
Takes: Heals 18 from cold, 0 radiant damage, and Damage Threshold negates the 9 Necrotic...overall HEALS 18 points per turn.
Hope that makes sense?