The Excellon format is widely used to drive CNC drilling and routing machines in printed circuit board (PCB) manufacturing. It is a variant of the EIA RS-274-C standard. Only the storrage of circles is possible.
The Excellon format can only store circles for a single layer. No layer information cen be stored. So always the active layer is stored or loaded. The handle of the load/attached/update command is identical to the Gerber format: load will add the file contents to a new cell, attached will load it into the current displayed cell, a update will replace the active layer in the current displayed cell.