Read below links it may help you clear your doubt
http://help.sap.com/saphelp_erp60_sp/helpdata/en/b1/c04204439a11d189410000e829fbbd/content.htm
http://help.sap.com/saphelp_46c/helpdata/en/ea/e9b6a14c7211d189520000e829fbbd/content.htm
the basic difference is create when you are setting up master data for the BOM
Reading is required when data is changed in the routing, in the work centers affected or in the BOM after order creation then these changes are not automatically taken into account in the production order.
Thus you can read in the master data (BOMs and routing) again into a created or released production order.
Best regards
Hrusikesh