on every bOM position for item a object dependency is written of type "selection condition" which select this component if the characterstic mention in dependency have exact value mention in dependency.
for more details on it you can use below link.
i recommend use below link to read more about SAP VC.
http://help.sap.com/saphelp_erp60_sp/helpdata/en/92/58d455417011d189ec0000e81ddfac/frameset.htm
Hope it will help you.
Thanks
Ritesh