Just think of standard wall or 'std' as any wall thickness in the same context as sch 10 or sch 20 or sch 30. The thickness or equivalence of 'std' varies with size.

For example 30" std wall is between sch10 and sch20, but 3" std wall is same as sch40 wall.

If you downloaded a file of pipe sizes you could easily have figured this for yourself.