With Ableton Effect Racks, effects can be made with simple building blocks. These blocks are the stock plugins or external VST or AU plugins.
According to the makers, “a Rack is a flexible tool for working with effects, plug-ins and instruments in a track’s device chain. Racks can be used to build complex signal processors, dynamic performance instruments, stacked synthesisers and more.” It is said to extend the abilities of even a single device by defining new control relationships between its parameters.
Read more about this here: https://music.tutsplus.com/tutorials/a-guide-for-creating-ableton-effect-racks–cms-30190