If the dealer has an Ace and 6-card, then this is a soft 17.
In double deck blackjack any hand 17 or above, the dealer must stand. When a dealer has an S 17, it means they have an Ace and a 6-card making 17. There are two charts depending on whether the dealer hits or stands on soft 17 (S 17). To play perfectly and win at double-deck blackjack, you should use basic strategy charts.
