Ash.Resource.Change.Builtins.manage_relationship

You're seeing just the function manage_relationship, go back to Ash.Resource.Change.Builtins module for more information.
Link to this function

manage_relationship(argument, relationship_name \\ nil, opts)

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Calls Ash.Changeset.manage_relationship/4 with the changeset and relationship provided, using the value provided for the named argument

For example

change manage_relationship(:add_comments, :comments, on_missing: :ignore, on_match: :no_match, on_no_match: {:create, :add_comment_to_post}