The "s" stands for stainless steel. It usually limited to 12" Sch 80. You will see larger sizes in 5s an 10s listed.
The spec used to be ANSI B36.19
The is sometimes a little confusion as there is STD weight pipe C/S pipe that is mistakenly called 40S.
Yes, as per above responses, suffix 'S' (w/o any other letter) stands for stainless. Available sizes given in Table 10-18 Properties of Steel Pipe, Perry's, 7th Edn.. Footnote:
5S, 10S and 40S from Stainless Steel Pipe, ANSI B36.19.
I wish to point out that S in combination with another letter is not stainless: ST = standard wall, XS = extra strong wall, XX = double extra strong wall, and Schedules 10 through 160 from Wrought Steel and Wrought Iron Pipe, ANSI B36.10.