In vanilla, multiple sources of resistances and weaknesses do not stack nor do they cancel each other out. The strongest source of resistance will override all weakness.
The selected effect is in the order of: Very Resistant > Resistant > Slightly Resistant > Immune > Very Weak > Weak > Slightly Weak > Normal
And below is default number for vanilla resistance and weaknesses
Very weak: 200% damage taken
Weak: 150% damage taken
Slightly weak: 125% damage taken
Neutral: 100% damage taken
Slightly resistant: 75% damage taken
Resistant: 50% damage taken
Very resistant: 25% damage taken
Immune: 0% damage taken
(https://valheim.fandom.com/wiki/Resistance)
For example, with this mod's default setting
RootArmor (pierce -20%) + BerserkerMead (pierce +30%) + BonemassPower (pierce -10%) = pierce 0% (Player would get 100% damage for pierce)
FeatherCape (Fire +45%) + BarleyWine (Fire -20%) = fire +25% (Player would get 125% damage for fire)
Mob's resistance and weakness also get affected.
For example, with this mod's default setting
Troll would get 100%+30% = 130% pierce damage (weak to pierce)
Bjorn would get 100%-20% = 80% from pierce (resistant to pierce)
## If on, the configuration is locked and can be changed by server admins only. [Synced with Server]
# Setting type: Toggle
# Default value: On
# Acceptable values: Off, On
Lock Configuration = On
[2 - Additive Damage]
## Very Weak modifier value. 45 means +45% damage taken. [Synced with Server]
# Setting type: Single
# Default value: 45
# Acceptable value range: From -100 to 100
Very Weak Percent = 45
## Weak modifier value. 30 means +30% damage taken. [Synced with Server]
# Setting type: Single
# Default value: 30
# Acceptable value range: From -100 to 100
Weak Percent = 30
## Slightly Weak modifier value. 15 means +15% damage taken. [Synced with Server]
# Setting type: Single
# Default value: 15
# Acceptable value range: From -100 to 100
Slightly Weak Percent = 15
## Neutral modifier value. 0 means no change. [Synced with Server]
# Setting type: Single
# Default value: 0
# Acceptable value range: From -100 to 100
Neutral Percent = 0
## Slightly Resistant modifier value. -10 means -10% damage taken. [Synced with Server]
# Setting type: Single
# Default value: -10
# Acceptable value range: From -100 to 100
Slightly Resistant Percent = -10
## Resistant modifier value. -20 means -20% damage taken. [Synced with Server]
# Setting type: Single
# Default value: -20
# Acceptable value range: From -100 to 100
Resistant Percent = -20
## Very Resistant modifier value. -30 means -30% damage taken. [Synced with Server]
# Setting type: Single
# Default value: -30
# Acceptable value range: From -100 to 100
Very Resistant Percent = -30
## Immune modifier value. -40 means -40% damage taken. [Synced with Server]
# Setting type: Single
# Default value: -40
# Acceptable value range: From -100 to 100
Immune Percent = -40
## Lower bound for final damage taken after additive sum. 0 means can go down to 0%, 50 means cannot go below 50%. [Synced with Server]
# Setting type: Single
# Default value: 0
# Acceptable value range: From 0 to 50
Minimum Damage Taken Cap Percent = 0