(as in it isn't one standard but a group of standards, like asn.1 without any encoding is split in ~4 standards by itself. Through to be fair all or CER, BER and DER are in the same standard. But PER is another standard, so is XER, OER, JER, GSER, RXER each and others.)

Practically, the useful encodings are DER, which is canonical and used for crypto, and XER, which is human-readable.

It is a neat spec, chock-full of great ideas. Unfortunately, given its age, there have been many bad implementations of it.

The TLS 1.3 "Selfie attack" is an example of a gap between what we did prove and what we intuitively understood.

The formal proof for TLS 1.3 says Alice talking to Bob gets all the defined benefits of this protocol, and one option is they have a Pre-shared Key (PSK) for that conversation. They both know the same key, in that sense it's symmetric.

But the human intuition is that we're talking about an Alice+Bob key, whereas the proof says this is an Alice->Bob conversation key. If we re-use the same PSK for Bob->Alice conversations too we get the Selfie Attack, the formal proof never said we can expect that to work, it was just our intuition which confused us.