These correspond to all the different options you have when creating mp4-compressed video - different image sizes, framerates, frame types, methods and limitations of arranging those frames, etc, etc.

To take the poster example further: posters can be various sizes, use various colours, be on different papers, have different borders, etc. It's not just a single format, but rather a description of the ways various things can be compressed. That leaves us with mp4, the compression format. Now, that out of the way, we can ignore the first one, the container type, for the time being. Think of it as the poster from the first point it's been designed to be carried around in a mailing tube - but, for convenience, often gets carried around in something else like a generic folio case (e.g.avi.

Secondly, there's the mp4 compression format.

posters), but not things that don't (e.g.

Think of it like a mailing tube you can put certain things in it that fit (e.g.

Firstly, there's the container format '.mp4'.

There's 2 different things that get called mp4.

Avidemux combine videos