From FFXI Wiki

Attack depends on Strength and the relevant combat skill with the weapon type currently in use. As of the December 2018 Update[1] Attack derived from Strength is in a 1:1 translation for all main-hand and ranged weapons:

Attack = 8 + Combat Skill + STR

For weapons in the off-hand, Strength contribution is reduced by half:

Attack (1H sub) = 8 + Combat Skill + Floor(STR ÷ 2)

Combat Skill translates into Attack in differing skill tiers, similar to Accuracy, and likewise includes Item Level skill bonus for the skill total even though said iLv Skill is not shown in the Skills menu.

Attack is used to calculate pDIF and determine the range of damages that are possible with any given base damage against a target with a specific defense and level.

Pet: Attack from Pet: STR

Pet's attack derived from their STR is via the same rate as a player's off-hand Strength derivation:

Attack= floor(STR ÷ 2)

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