[ECI-EN] PSO?

What does PSO stand for? The acronym is not defined in the "PSO_Coated_v2_300_Glossy_laminate_eci_info.pdf" or the eci web site, but it's used in a pile of profiles from 2009, on the eci website. At best it's correlated to a description: conforming to ISO 12647-2, PT 1/2, 150–200 lpi screen, in one document. Yet in another document for PSO INP Paper, this is totally contradicted. So I have no idea what PSO means. Thanks, Chris Murphy