An Alpha mask is included in Basemap quads or tiles.
The alpha mask is an additional band provided as a binary color channel, therefore representing binary values. The alpha mask can be used to render, remove or hide low-image-quality pixels near the periphery of a given scene. It compensates for effects due to vignetting, low SNR, or hot or cold pixels.
You may reference our Product Specs as well: https://www.planet.com/products/satellite-imagery/files/1610.06_Spec%20Sheet_Combined_Imagery_Product_Letter_ENGv1.pdf