It looks like you can do either depending on how many shipments you want to create. 1 shipment - just populate PLI...if you need multiple shipments created, then you need to specify that information in PLLI and set the line_loc_populated_flag to 'Y'
if you had the case where you wanted more than 1 shipment created for the line, then you would use the PLLI table (ex d)...all the other examples are just inserting into PLI (a,b,c,e).
Vision Demo - How To Import Standard Purchase Orders Using the Purchase Document Open Interface PDOI - Pre PDOI Enhancements (Doc ID 1054004.1)
This note may help too since it specifically mentions the flag...Set to Y when you are specifically populating PLLI else N.
Vision Demo - How To Import Standard Purchase Orders Using the Purchase Document Open Interface PDOI (12.2 and 12.1.3 Enhanced PDOI) (Doc ID 2014101.1)
After section a - Also note that the PO_LINES_INTERACE.LINE_LOC_POPULATED_FLAG is set to N - as PO_LINE_LOCATIONS_INTERFACE is not being populated.
Hope that helps!