DATEDIF(), which means Date + Dif, is a compatibility function left over from Lotus 1-2-3 that Microsoft adopted in Excel version 2000, which is the only version that explains how this function works.
=LET (Spend,SUMIF (T_Budget [Category],E2,T_Budget [Cost]),IFS (Spend>F2,"Over budget",Spend=F2,"Budget hit",Spend> (F2*0.9),"Near budget",TRUE,"Within budget")) Let's break the formula down to ...