The average includes all candidates that FiveThirtyEight considers “major.” Candidates with insufficient polling data are not displayed in the averages. In June 2023, they debuted a new way of calculating our polling averages.You can read about their methodology here.
This market resolves with “Yes” if at 00:00 on 1st January 2024 (UTC), National : President: Republican primary : 2024 Polls | FiveThirtyEight report that the likelihood that Trump wins the Republican nomination is above 45%. It resolves “No” otherwise. If 538 removes their poll average, then another credible source will be used.