Answer

"S/K" LED indicates the installing CPU is Skylake-X or Kaby lake-X, it lights red when installing Skylake-X CPU; lights white when installing Kaby lake-X CPU.

Reference

Models

X299 TOMAHAWK, X299 TOMAHAWK AC, X299 GAMING M7 ACK, X299 RAIDER, X299 SLI PLUS, X299 GAMING PRO CARBON AC, X299 TOMAHAWK ARCTIC, X299 GAMING PRO CARBON, X299 XPOWER GAMING AC