The "damage" tag used for tridents does nothing. Ideally it should dictate the exact amount of damage the trident deals.
Test Data:
In survival or adventure mode, run the following commands and see how much damage each trident does:
/summon trident ~ ~5 ~ {pickup:1b,damage:2d}
/summon trident ~ ~5 ~ {pickup:1b,damage:5d}
/summon trident ~ ~5 ~ {pickup:1b,damage:10d}
Expected Results:
The tridents deal 2, 5 and 10 damage (1, 2.5 and 5 hearts) respectively.
Actual Results:
Each trident deals 8 damage (4 hearts)
Additional Information:
Naturally spawned tridents always have a damage value of 2. This is the same as the default for an arrow. If the bug is fixed, the default damage value would need to be changed to 8 to keep the trident at its usual strength.
The trident also has the crit tag from arrows, but I haven't yet tested whether this is functional.
Although I only posted it during 1.13.1, this issue has likely been in the game since tridents were first added.
The trident calculates the damage it deals based on its enchantments with a base value of 8. The "
damage" tag is only used by arrows to save additional damage added from the Power enchantment (EntityTridentextendsEntityArrowso it inherits the tag).