those 18 inch rims are the ones that come with the Sport package option. Along with the rims, you get lower and stiffer suspension (sport suspension).They are not standard on the S-line.
S-Line includes the Sport package. From the Audi website:
S line package (contains Leather seating surfaces)
Sport suspension
Brushed aluminum decorative inlays
Leather-wrapped three-spoke perforated steering wheel and shift knob
Shift paddles on steering wheel in Tiptronic models
S line door sill trim
Aluminum optic pedals in manual models
18" Five-spoke cast alloy wheels with all-season tires (2.0TFSI)