OPTIONAL flag under Bill of Material setup

Hi All,

I have a PTO item "TP-LCNUPG" (model:TP-LCNUPG and sub-component:LCN-NONSESP) whose main item is defined as MODEL item and sub-component defined as STANDARD.

In BOM setup under ORDER MANAGEMENT tab we have sub-component which has OPTIONAL flag has checked.
My question here is how can we uncheck the OPTIONAL flag for the sub-component item.

Bcoz when ever i am unable to check from the application as it is saying "Field cannot be updated".

Please give us a hint or any solution which will help me.

Also please provide me a document for creation of PTO option class, PTO model and as well as PTO kit items (I am not aware if these are all different or one and the same, please clarify)