When you create a new field and select “Lookup” for Type, it gives you only one option to select for its Related Record Types. It’ll be nice to have more flexibility in creating the lookup field, enabling users to select multiple data types for reference.
A good example is the required/optional attendee fields in Activity entity when creating a new appointment: it gives you several types of records to pick from “Look For”, whereas custom field is restricted to only one type of record.