authenticators property
Authenticators is a list of authentication handlers that will try and authenticate the provided credentials. Authenticators are checked iteratively from index 0 to n and if the first authenticator to return a positive result will be the one used. If you want the rule to first check a specific authenticator before "falling back" to others, have that authenticator as the first item in the array.
Implementation
@BuiltValueField(wireName: r'authenticators')
BuiltList<RuleHandler>? get authenticators;