MARC21 is merely the most widely accepted flavour of MARC in the USA.
Some other, arguably more important flavours are UNIMARC and INTERMARC.
Perhaps best that can be said about MARC-21 is that it is backward compatible.
MARC-21 is rather complex, but I think you may enjoy a look at MARC-8, which is not a predecessor of but a part of MARC-21. I am guessing that substandard will look somewhat familiar to you. A brief look at Unicode support in MARC21 should convince you that this is a standard you use because you positively have to.