All images, warps and stacks, have Pixel Flags set for each individual pixels. This information is saved in a mask image, which in general has the suffix .mask.fits(.fz) and mk.fits(.fz) for warp and stack images, respectively.
The starting point for the PS1 data archive is at Pan-STARRS1 data archive home page.
The following information is taken from Waters et al., which should be cited appropriately.
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