Acrylic comes in two forms: Cast and Extruded.
Cast acrylic is used for almost all engraving purposes because when it is engraved it produces a white, frosty look that makes for a nice contrast against the clear material.
Whereas extruded acrylic remains clear when engraved. For this very reason extruded acrylic is ideal if you are only going to vector cut.
The material has a lower melting point, which produces an almost flame-finished edge when cut with the laser, much easier than compared to cast acrylic.