If users are Using Order Entry, can comments be added to orders?

Yes!

Once an order has been submitted by an Order Entry user it will await the approval of a Traffic level, or higher, user.  If the order is accepted, then it will be posted to the system and go into rotation.  however, if the order is rejected, the Trafficker can add notes to the campaign so that the order entry user knows why it was rejected.  In most cases, orders will be rejected for missing specific parameters.  The Order Entry user can use the information from the Rejected Note to make any necessary edits or modification before re-submitting the order.