The nomenclature here is based on firmware 0.4.25.
A container is just basically an empty shell Unit. A place for you to start creating a chain of units that you’ll likely want to re-use again and again, or even share on the forums. Say you want to create a synth voice, or some effect. You might start with a container unit so that you have something there besides a chain to save it all into.
They aren’t quite like a mixer, because a mixer actually does something on it’s own (mixes the signal coming into it with the signal in its subchain). A container doesn’t do anything until you fill it up with some Units.
The Custom Source unit blocks any input coming into it from the left. It would be suitable for a sound generator. Try placing a Custom Source unit into a chain. Then put a Sin Oscillator in front of it. You’ll notice the sine wave signal disappears and shows No Signal at its output because the Custom Source has blocked it. You’d want to place your sound generating units inside of the Custom Source rather than in front of it.
Now replace the Custom Source with a Custom Effect, and notice that the sine wave passes through it. So this kind of container is more suitable for an effect, like, say a delay or whatever - anything that’s going to process an incoming signal rather than generate its own signal.
There are also some special ones for multi-band. They’re a little more advanced, but they essentially have multiple signal paths that get mixed back together at unity gain before going to the output. You could use these for effects that process a signal in different ways in parallel.
Of course you can rename these containers, as well as add custom controls to them that control the params of the Units inside of them. That way hopefully you (or anyone you share your unit with) only has to pay attention to the top level controls, and can ignore the whole sub-chain of Units inside that makes your creation do whatever it does. Many (most?) of the unit presets tagged “custom-unit” on this forum were built inside of a container.
Hope that helps - feel free to ask for clarification.