To build off of this users idea (https://community.esri.com/t5/arcgis-field-maps-ideas/better-attribute-rules-error-descriptions-in-f...)
1. Agree with everything in the idea above - error messages defined in tables/feature class showing in the field map app
2. Refinement of constraints and how they work with the field app. Sometimes the constraints work in the field app and sometimes I will add the same constraint to another layer and field and it doesn't work. The code needs enhancements and updated documentation for field maps on this topic.
3. Adding the view mobile application from field maps designer like you can in Survey 123 so you can confirm these error messages and constraints.
4. Having field maps app through web like Survey 123 so testing can be done on web and not personal devices. (understanding that GPS settings cannot be recognized on web app
