Chicago's Smyth 20% "service charge" covers what, exactly?

https://preview.redd.it/8c19vcpp0czd1.jpg?width=1086&format=pjpg&auto=webp&s=3482c9309b1435d156ffb0e99ed0ea86769cdc4e

This was included in the e-mail prior to our Saturday meal.

Am I to understand that I will be paying a 20% fee on top of a very expensive meal and that I will *also* be expected to provide a gratuity?

I'm confused...