Security rules

Hi

I have a question regarding security rules. this question is regarding the purchasing and inventory user's restirctions.

Some account are default and picked by the system automatically, if i give the user only the expense accounts access and not provide the liabilities accounts access that system picks automatically.

Can User will do a transactions or system allow him to perform the transactions.

Further, for Assets transaction do i need to give access to the user for all clearing account or system will allow him to do transaction without having the access.

thanks

Haider