Best field definition which is only use for filter query.

Below is the sample document *{"filedA": 1,"filedB": "","filedC": "Sher","filedD": "random","rules":[203,7843,43,283,6603,83,513,5303,243,103,323,163,403,363,5333,2483,313,703,523,503,563,8543,1003,483,1083,2043,6523,603,963,683,5353,763,443,643,743,723,1123,843,1243,1663,1803,1403,1783,7563,3

Why External File Field is marked as indexed in solr admin SCHEMA page?

We have following external file field type and field: ** ** In solr official documentation is it mentioned that : *"*The ExternalFileField type makes it possible to specify the values for a field in a file outside the Solr index. *External fields are not searchable. They can be used only for fun