This is a 3D profile drawing of one end of a typical trench type drainfield. As you can see, the monitoring tube extends below grade and all the way to the bottom of the field. This way, the effluent level can be monitored simply by removing the cap on the top of the standpipe. The cleanout only extends down to the distribution line. The distribution line is where the effluent, that comes from the septic tank, is dispersed throughout the drainfield.
This is a 3D profile drawing of one end of a typical trench type drainfield with a sump. Sumps are a combination of the cleanout and monitoring tube. During an adequacy test, the liquid level in the drainfield is measured through the sump.