The layer 21 & 22 (tplace & bplace) are the silkscreen layer. These two layers contain both the top and bottom silkscreen on your PCB and also component outlines to show the positioning of components. You need to be careful when using this layer not to place silkscreen on any soldered areas.
As an alternative to these layers, consider placing additional silkscreen on Layer 51: tDocu for your own personal reference like comments in programming. This layer won’t be included in your manufacturing data or printed on your PCB so that you can include a lot more details. However, if you want to add any kind of additional artwork outside of regular silkscreen, like text or logos, then use Layer 21-22 as I mentioned earlier.
Let me know anything more you need to know!