This value is the average of the dividend Yield over the last 60 months (approximated using 21 bars). Up to 60 samples of Indicated Annual Dividends (IAD) are used to calculate 60 yields, then averaged.

This is very similar to the value computed by the FHistAvg function which also spans 5 calendar year but uses 65 samples of Yield every 4 weeks.

FHistAvg("Yield",65,4)