It is quite common for the alignment of scanned paper documents to vary. While OCR can correct for rotation, the horizontal and vertical position will often fluctuate from page to page.
Anchors are special types of reading zones. They look for prominent geometric features on documents, such as page corners. After these features are found, anchors can provide a fixed coordinate system for the reading zones. Position for reading zones is determined relative to anchor positions. This can overcome fluctuations in the page position of scanned images.