One aspect of creating decision services is easing the burden in the algorithm that determines the root of the request data.

If the root Entity does not have a defined #type in the __metadata (both are optional), then an algorithm is applied to find the best match between the data in the payload root Entity and that in the Vocabulary.

The Root Entities setting allows a user to define a set of possible Entities that the algorithm will be applied to. This will limit the number of Vocabulary Entities to be compared against the payload Entity. Having the algorithm only compare a limited set of Vocabulary Entities, the matching may be more accurate, and with less processing time, and faster execution. If no Root Entities are defined for a decision service, the algorithm will need to compare each root Entity in the payload against all Entities in the Vocabulary to determine best match.

You can multiselect in this dialog.

It is important to note that the selection is part of the Decision Service. If you have entities A, B, C, D, and you chose C and D as the Root entities, the algorithm will only assign the Root entities to C or D, even if the root Entity in the payload better matches against an A or B. If you make no selection, the algorithm will make best effort.