The big difference between an analog modular synthesizer and their digital brothers and sisters is the ability to store sound presets. That is something that is not possible in an analog modular. There are of course some hybrid analog synthesizers, like the Buchla 200e, where (a big part of) a patch can be stored digitally.
However, although you cannot store a patch on an analog modular it would be nice to be able to document it. Fortunately, Bill Sequeira and Axon Hillock already thought about that and came up with MPN: Modular Patch Notation, an easy and flexible way to describe analog modular synthesizer patches.
Read “MPN – Modular Patch Notation explained” for a detailed description.