Abilities
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Some abilities require you to have other abilities, attributes, or skills before you can acquire them. You cannot increase the ability tier above the prerequisite attribute modifier or skill proficiency bonus. Some may say "or" allowing you to apply one.
For example, Sneak Attack has the following prerequisites: Dexterity, Finesse, Marksmanship, Stealth.
Dexterity is the only attribute so that serves as the attribute prerequisite.
Finesse, Marksmanship, and Stealth are all skills so you'd select one of those. Say you selected Stealth.
So, the highest tier in Sneak Attack you'd be able to gain would be the lower of your Dexterity bonus or your Stealth proficiency bonus. If either of these were 0 or less, you'd be unable to gain Sneak Attack as an ability at all. If your Dexterity were +3 and your Stealth proficiency bonus were +4, the maximum tier you could increase Sneak Attack to would be 3rd, as the lower of these two numbers is 3.