PXE Advertisement Types

Mandatory advertisement:
     Computers that receive a mandatory advertisement that is enabled for PXE will boot using PXE without any user intervention. The user will not be given an option to bypass the PXE boot. 

Note: If a user cancels the PXE boot process before the PXE service point responds, the computer will not receive the advertisement.

Optional advertisement:
     For computers that receive an optional advertisement that is enabled for PXE, a user must be present at the computer to press F12 to continue the PXE boot process or the computer will boot into the current operating system or using the next available boot device.

Clear Last PXE Advertisement:
     If for any reason you want to re-advertise a mandatory advertisement enabled for a PXE device or assigned to a collection you can select this option by right clicking an existing Configuration Manager 2007 collection or a device with an existing PXE advertisement and selecting ‘Clear Last PXE Advertisement’.

Important: Selecting this option will reset the status for the last mandatory PXE advertisements for the collection or device. The most recent mandatory advertisements will be rerun on any computers or devices the advertisement has been assigned to.

http://technet.microsoft.com/en-us/library/bb680753.aspx