If you are submitting gerber files converted from eagle cad software, and you are not sure which one to select on the layer mapping screen during the order process. Below is a list for those file extensions mapping to different layers:
.cmp cmp is component side copper (TOP)
.sec .Sol (Solder Side)
.smc .Stc (Solder Stop Mask, Top Side)
.sms .Sts (Solder Stop Mask, Bottom Side)
.ssc .plc (Component Side Silk Data)
.sss .pls (Solder Side Silk Data)
.drl .Drl (Excellon Tool List)
.drd Drd. (Excellon NC Drill file)
.dri Dri. (Drill Station Info File)
.gpi gpi is gerber plot information
.brd brd is eagle board fiel, cad tool (eagle native file format)