Differences training for tailwheel is entered in your logbook and is not a rating in itself, although it forms part of the relevant class rating.
FCL.710:
The differences training shall be entered in the pilot’s logbook or equivalent record and signed by the instructor as appropriate.
Or, CAP 804, Section 4, Part H, Subpart 1, page 2 if you prefer the CAA website:
Differences training | Private Pilots | Personal Licences and Training
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