This is a small tray to hold screws, nuts and other small parts. The bottom has 3 embedded 12x3 magnets (Amazon 12x3 Magnets).

The slicer has a PAUSE at Layer #22. This is when you insert the 3 magnets into the holes. When you resume printing, the magnets will be covered.

If you have a magnetic bed it is recommended to observe the polarity of the magnets when you insert them to make sure that they stay in place while they are covered. Even if your bed is not magnetic, if you print several of these trays, keep the same magnet polarity so that the trays can be stacked.

Even though the magnets are held in the holes by friction when you insert them, it could be a good idea to add a small drop of Crazy Glue at the bottom of the holes. A very small drop so that the glue does not ooze out when you insert the magnets.