Skills alongside Advancements was one of the original lock types added by Skillable and has since then been refined to support other mods adding more Skills. The Skill Syntax is as follows:

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Example:
ResourceLocation|Level

Working Example:
reskillable:building|15

Since the release of Reskillable we've moved from a skill name system to a resource location name system. This allows for several skills to add skills with the same internal name but different mod id's for the Resource Location. You can find the Skill's ResourceLocations under: Skills & Traits