office machines for automatic envelope bend (puts the sheet in
envelopes) needs the paper (letters, invoices and so on) printed with
some special marks: Optical Mark Recognition (OMR).

here is some websites where you can found these machines:
http://www.skywell.com/folder_inserters/folder_inserters.htm
http://www.infordata.it/english/automailer1en.htm

My question is how are defined these marks, exists some "standard"
about how these marks must be printed? sizes, colors, and so on?.

My tasks is be able to print these marks in the documents. My program
can print rectangles, images, shades, and so on anywhere in the paper,
then i need to know what types of "objects" or drawings are compatible
with OMR for envelope folding.