A: We need the following files for every pcb assembly order:

PCB assembly BOM list (or .xls spreadsheet). The BOM needs reference designators, value, package/decal, description, and part number.
Cad-generated component location XYRS file-also called the parts position/rotation file or the machine pick-n-place file. Each CAD system will generate this, if requested.
Board X,Y, 0,0 reference designators; Board Fiducial Marks are appreciated but not required.
Cad-generated Gerber files for Top/Bottom Copper, Silkscreen, and Solder paste mask. We also need the solder mask file if we're buying the PCBs for you.