What is the meaning of B1 or B2 format press ?

What is the meaning of B1 or B2 format press ? is it specification of machine size or others information about press. please describe...

 

These are paper sizes.
B1 format presses can handle a max paper size of B1 format and are usually 28' x40' inches in size.
B2 format presses are usually 19' x26' Inches.

 

Below are the paper sizes that can be handeled by B1 or B2 presses.
B1 707 x 1000 mm 27.8 x 39.4 in
B2 500 x 707 mm 19.7 x 27.8 in

 

I'm pretty sure a Heidelberg SM 102 can handle a 28"x40" sheet size, in fact I know it can because I ran one.

 

Yes it does but max print area is smaller.

 

I don't think the OP was referring to maximum print area? He asked what is the meaning of B1 / B2 as per press. an SM 102 can handle B1 which is as you say 707mm × 1000mm or 27.8" × 39.4" but the SM 102 which is the classic B1 press can still handle a 28"x40" sheet size, the clue is in the name 102.

 

I posted the standard B1 and B2 size as the OP didn't ask about any specific manufacturer. He can refer to the websites of any press manufacturer if he's looking for any specific brand.