12FS8 is a rooted blonde.
The original color code for this was 12/24B/102 -- a buttery golden blonde as the top base shade with platinum highlights and a light golden brown nape; the medium brown roots were added later and provided the “S8” part of the code.
Since 12/24B/102/S8 is a mouthful, they shortened it to 12FS8. (The version without the dark roots was shortened to just 12F.) 😊
You can watch Heather's in-depth color spotlight on this shade here: