If you are submitting gerber files converted from Protel altium 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:
.gtl top copper
.gbl bottom copper
.gts top solder mask
.gbs bottom soldermask
.gto top silkscreen
.gbo bottom silkscreen
.txt excellon drill file
.drr NCDrill file report
.drl NCDrill file
.apr aperture list
.gko keepout layer, sometimes board outline
.gp1 ground/power 1
.gp2 ground/power 2
.gtp top pad master
.gpt top paste mask
.gpb bottom paste mask
.gbp bottom paste mask