Don’t worry too much about this as you will not see it in the exam and only ever see it in the study materials. The X and W relate to the decade effectively, so 20W0 to 20W9 is one decade and the next one would be 20X0 to 20X9, as X comes after W in the alphabet.
In each decade the years then increase as normal so 20W1 is after 20W0, and then comes 20W2.