This looks like returning a list of contacts. In that case you might have some base Contact class that has a reference number as sole property. There are 2 subtypes: Company and Person, having their own properties. entity_type is used as the discriminator and tells the json deserializer which class is to be constructed. So yes, this is normal.

In a way, this is not that different than writing a single flat object with all possible fields and just omitting the ones that have no / a null value.