Notes on Annotations:
@Angle, @Color, @Distance, @Percent
If present in the collection, they have no value @Hidden
Attributes annotated with @Hidden are not retrieved from the Rule Package.
@File and @Directory
Attributes annotated with @File or @Directory are not retrieved from the Rule Package. They are not supported for renderers. @Handle
@Handle is not retrieved. @In and @Out
@In and @Out are not retrieved. @In is reflected in the RulePackageDescription.GeometryType. An @Out is implicitly always combinedShape. A value of separatedShapes for @Out is not supported. @Group with index
When an order index is added to a @Group ~that~ group order index is added as a GroupOrder key/value.
@Range
@Ranges can be numeric or a domain of specific values. Check the RangeType key. It will be one of "Range | Domain".
@StartRule
@StartRule is not retrieved.
Annotation keys:

• Angle : if present indicates attribute was annotated with @Angle. No value (empty string)
• Color : if present indicates attribute was annotated with @Color. No value (empty string)
• Distance : if present indicates attribute was annotated with @Distance. No value (empty string)
• Description : if present indicates attribute was annotated with @Description. The description, if any
• Group : if present indicates attribute was annotated with @Group. A string containing the comma-separated group values
• GroupOrder : if present indicates the @Group contained an index. An integer 0 or greater
• Order : if present indicates attribute was annotated with @Order. An integer 0 or greater.
• Percent: if present indicates attribute was annotated with @Percent. No value (empty string)
• Range : if present indicates attribute was annotated with @Range. A string containing the comma-separated range values
• RangeMin : if present indicates the @Range was numeric, RangeType = "Range". A double, the minimum range value
• RangeMax : if present indicates the @Range was numeric, RangeType = "Range". A double, the maximum range value
• RangeType : if present indicates the original @Range type. One of "Range | Domain"

System.Object
   ArcGIS.Desktop.Mapping.RulePackageAttribute

Target Platforms: Windows 11, Windows 10

Reference

RulePackageAttribute Members
ArcGIS.Desktop.Mapping Namespace