Design files for mirror acrylic should be mirrored, otherwise they will be fabricated back to front. So don't forget to flip your design across the Y Axis before uploading it to your Personal Factory.
By default, the material will be placed coated side up in the laser cutter, so any engraving will be on the coating. The engraving will remove the reflective coating making that section of the acrylic translucent.
If you want to engrave on the gloss side, make a note in the shipping instructions for your order. Engraving on the gloss side can only be done through the protective backing paper, so only heavy raster and vector engraving are suitable.