Some field types index multiple values per field, for example Image field type, which indexes file name, file size and alternative text. One of these is default, used by default by Field criterion and sort clause. Others can be accessed through Field criterion and sort clause implementation of CustomFieldInterface.
ATM field type search integration test framework makes testing of these, custom fields a bit awkward for new implementations. Test framework needs to be refactored to make testing of these custom fields easier.