As an industry standard, PCB manufacturers recommend customers provide an IPC-D-356A net list file with their gerber data files.

Advantages of Providing an IPC-D-356A Net List File

• The IPC-D-356A net list file is generally extracted from the CAD system at the same time as the gerber data extract so the the files match.
• Upon receipt of both files at the PCB manufacturer, the IPC-D-356A net list files is immediately compared to the gerber files to confirm the gerber files received are complete and accurate. Any differences will be identified and questioned back to the customer. 
• During the CAM tooling stage at the PCB manufacturer, the net list file will operate in the background to help prevent the CAM operator from making a functional change that will impact the electrical design of the PCB. 
• During the electrical test stage at the PCB manufacturer, the IPC-D-356A net list file will be used to confirm the PCB manufactured electrically correlates to the gerber / net list file received.

Disadvantages of Using a Gerber Extracted Net List File

• The OEM will lose the assurance of gerber data accuracy.
• A gerber extracted net list at the PCB is only as good as the actual gerber at the time of net list extraction. For example, if a feature was accidentally altered during transmission, translation and/or during the gerber extraction of the net list, the defect would be embedded into the gerber extracted net list file; which would then be used for CAM panelization and electrical test.