Level sensors are designed to determine the distance to a surface. In a container, this translates to the level of the substance.
A sensor such as these can also be used to determine position.
Level sensors can be used in a continuous mode, sensing variable level in a continuous manner. They can also be used in a ‘point’ mode in which they sense the presence or absence of the substance at a specific point.
Continuous level sensing can be performed from the top or from the bottom of the vessel. When measuring from the top the ultrasound must propagate through air or whatever gas is above the material being measured. Because of propagation limitations through gas there can be significant limitations on the distance or resolution of the measurements. When propagating up through liquid rather than down through the air, the distance and/or precision is generally better. There may be issues with access to the bottom of the vessel or other mechanical constraints which limit this method. If measuring the level of a granular solid material, coming up from the bottom may not work at all.
Point level sensing is often all that is required. This is analogous to a mechanical limit or float switch. The presence or absence of liquid can be very reliably determined so either the upper limit (to stop filling, for example) or the bottom limit (say to start filling) are possible.