NYSE Arca has amended its rules to add a new order type, the Primary Sweep Order (PSO), which is a variation on Arca’s existing Primary Only order type (PO Order). Unlike the existing PO Order, which must be entered before 6:28 a.m. Pacific Time and is restricted to participation in the primary market opening without first sweeping the NYSE Arca book, a PSO may be enteredat any time and will sweep the NYSE Arca book before routing to the primary, listing market. Unless a PSO is designated as immediate-or-cancel, the routed order (or the unexecuted portion thereof) will remain on that market until executed or cancelled. A PSO may be designated as an intermarket sweep order, subject to the provisions of Regulation NMS.

http://a257.g.akamaitech.net/7/257/2422/01jan20071800/edocket.access.gpo.gov/2007/pdf/E7-11719.pdf