This tag indicates that an image sourced from pixiv is sampled, and is not the original image.
Pixiv has a variety of 'sample' versions of images, in which the original has been automatically resized/modified. Because pixiv's process doesn't always do a good job, these versions can even have larger filesizes, but they are nonetheless not the original image and should not be uploaded here.
This tag applies to any post whose MD5 hash matches a sample size but not also the original size. Do not add this tag unless you can confirm a difference, or remove it unless the hashes match.
The URL of the original size contains '/img-original/' and just the page number as a filename suffix ('_p0', ...). Samples contain '/mobile/', '/img-master/', and/or various filename suffixes ('480mw', '128x128', etc...). Pixiv used to use a different URL system, with '_big_p#' to indicate full manga-mode images and sample indicators like '/img-inf/', '_m', and '_s', but those are no longer in use.
The original sizes of manga-mode images are accessed through small icons next to each page if you are logged onto pixiv.