These custom field views connected to two different standard views purchase_order_line_pub and purchase_order_line. Normally the view for purchase_order_line_tab will be purchase_order_line. However there can be alternative views that ends with _UIV, _PUB, _BI, _LOV, _LIST - refer to post https://community.ifs.com/technology-all-about-the-nuts-and-bolts-that-make-it-work-50/database-view-uiv-486#post1748 as well. 

_pub views are not automatically generated, rather they are created by IFS developers purposely. Most probably the base view purchase_order_line and the alternative view purchase_order_line_pub is being used in two different IFS screens with their own purposes. 

Hope this helps.