hi Swadesh,
Have a look on links below, regarding lock Mechanism...
http://help.sap.com/saphelp_46c/helpdata/en/c2/2d7037ecc92a7ee10000009b38f8cf/content.htm
http://help.sap.com/saphelp_nw04/helpdata/en/41/7af4c5a79e11d1950f0000e82de14a/content.htm
Thanks
A.Kumar