Cleanout hatches are usually only found on high-recovery commercial water heaters. As the name implies, it is a gasketed port that can be opened in order to remove sediment buildup. This can be done a handful at a time or with a shop-vac or Muck-Vac with special attachments.
If done by hand, it’s helpful to have small hands, since the port isn’t very large and a closed fist barely passes. By nature, there are other limitations to its use, since it is difficult to reach to the far recesses of even a simple one-flue tank, and impossible on a multi-flue water heater.
The black panel in the photo at right covers one such hatch. They are usually found low down on the side of the tank.