This tag indicates that an image sourced from ArtStation is sampled, and is not the original image. An ArtStation sample can either have lower image dimensions, or have the same dimensions but truncated bit-by-bit quality.
ArtStation formerly had a much more complicated image sampling issue, which can be referred back to by looking at previous revisions of this article or viewing the legacy procedure for avoiding samples at howto:artstation. Now a user may errantly upload from image URLs including "/small/" or "/medium/" rather than the correct "/large/" which would be replaceable by a moderator or by uploading the unsampled version and manually deleting the sample. There was formerly a fourth "/original/" which was manually accessible to save bandwidth. ArtStation terminated access to such files reportedly also to save bandwidth.
This tag implicates image_sample (learn more).