When you click on a reference selector, there is a tab ‘Selectable objects’ when you can set this restriction
That should do the trick, good luck!
Depending on your domain model you could also use the option: ‘Constrained by’ for this.
Here is some more information on it in the documentation.
Yes there is. If you open the reference selector edit screen and go to the selectable objects screen you can either set an Xpath constraint there or you can create your own data source microflow which returns the list of objects you want selectable.