When two liquids cannot be mixed, but separate from each other, this is called an interface. Oil and water are good examples of this phenomenon. Oil floats on water, which leads to the development of a separation layer at the transition point between the two liquids. The interface between two liquids is the first one of two varieties of interfaces.
The other one is solids that do not dissolve in liquids, but settle to the bottom, i.e. sediments in liquids. One example that can be cited is the sludge layer in a liquid.
There are different solutions for monitoring these two types of interfaces. Both, point level sensors and level sensors, can be used for this purpose - depending on the application.