A coil can not have two bonuses with the same effects.
Tertiary effect can only be an Elemental damage bonus.
Secondary effect can only be Damage, Tear or Handling bonuses.
Spear Damage Coil always provides a single damage bonus effect with a maximum value of 30%.
Handling bonus has a maximum value of 67% if it is the primary effect (=Handling Coil) or a maximum value of 33% if it is the secondary effect. It can not be rolled as a tertiary effect.
These bonuses can not be rolled as a tertiary effect.
Damage and Tear bonuses have a maximum value of 50% when it is the primary effect and a maximum value of 25% when it is the secondary effect.
Elemental damage bonuses can not be rolled as a secondary effect.
Elemental damage bonuses have a maximum value of 50% when it is the primary effect and a maximum value of 17% when it is the tertiary effect.
In the base game, the minimum modification value for all weapons coils is 6% and the maximum value varies depending on the effect and its "rank" on the mod: Coils are modifications that can be added to weapons to increase damage dealt or handling stats.