I think it just means that if you took the test after 17 December 2019, you will have a reference number instead of a paper pass certificate.
On 17 December 2019, they changed from issuing the result as a physical certificate which you had to scan and upload, to issuing the result as a Unique Reference Number which you type into the application form.
I don’t think the specific format of the reference number matters, just that you have a reference number rather than a certificate.
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