Iceberg order
Showing only a small part of a large order in the order book, refilling it as it fills. One of the technical mechanisms of absorption.
Institutional buyers use icebergs to hide their intentions; on the DOM it shows up as a small order that keeps refreshing at the same level. On the footprint, unusually high volume builds up at that level.
A retail trader can't detect an iceberg with certainty, but the observation that "the level is holding and volume is piling up" is a good enough clue.