You're seeing just the function persist, go back to Ash.Dsl.Transformer module for more information.
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persist(dsl, key, value)

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Saves a value into the dsl config with the given key.

This can be used to precompute some information and cache it onto the resource, or simply store a computed value. It can later be retrieved with Ash.Dsl.Extension.get_persisted/3.