Thanks for the reply, Luis.
Its not about making field as mandatory in windows. Its about enabling Dimension as mandatory in Accounting Schema ->Dimensions.
IDempiere has 4 custom dimensions user1_id, user2_id and so on. There is a checkbox on dimensions tab to make dimension as mandatory so when any posting happens in the system, that dimension is mandatory to be either set from transaction or default from the dimension tab.
Mandatory checkbox to be checked is allowed for Activity, Business Partner, Project and Product however it doesnt allow to make any of these custom dimension as mandatory.
I am looking why is it not allowed and if we need to allow it, would there be any issue?