Use Dynamic Array Output of Excel FILTER Function in Include Argument of Another FILTER Function
Solution 1:
Use ISNUMBER(MATCH()) where the values to search is HelperTable[SEQ] and the array to be searched is the return of your first filter: FILTER(HelperTable[SEQ],HelperTable[CATEGORY]="Fruit")
=FILTER(DatTable[VALUE],(DatTable[DEPARTMENT]="Blue")*(ISNUMBER(MATCH(DatTable[SEQ],FILTER(HelperTable[SEQ],HelperTable[CATEGORY]="Fruit"),0))))
