This is to keep your projects at the same spot on the laser bed, you can tighten one screw or both for a really secure hold. remember to adjust focus and laser before you put these in place.

this fits my rails which are 5mm thick and 15mm high and my bars are aprox 55 mm apart.
added Epic Laser Corners3 is for rails that are different than mine and you can tighten only have one side tight

Creality

Ender 3

No

No

200

38