I assume you mean ASME B36.10M and B36.19M. The suffix is for metric. have you read the foreword? Most people dont, but I advice to always do this for any standard you use. It will probably answer your question.
The main difference is that .19 was originally for SS pipe, and the other is for wrought stieel pipe (and more related to CS pipe), as the title says.
However, theyre yused interchangeable where possible, i.e. you can order e.g. ASTM A312 316 pipe to schedule 160.
At certian sizes, ASME B36.19M sch80S differs from ASME B36.10M; see note (2) in table 1 of B36.19M, and hence it matters if you specify an ASTM A316 316 12" Sch 40/40S to B36.10M or B36.19M.
