I plan on asking a question that goes something like: if you're an IT Pro what RFC numbers should you have memorized?

Example

In the SMTP/Sendmail community it is common to say RFC 821, or 2821 when talking about the envelope of a message, and 822, 2822 when referring to the envelope. This was even expected of me during an interview at Goldman Sacks, Microsoft, and Pfizer. Why? because there are two headers: a secret one and the other one that's visible when you view headers in an email/ Knowing the numbers expedites communication when differentiating between the two.

I'm sure there are other IT shops that rely so heavily on an RFC standard that they have incorporated this into regular parlance.

Question for meta

I expect this to be a finite list of distinct constructive answers that are valuable. That makes me think it is On Topic and Constructive. Do you agree?

My next question is should it be CW or not? I want to incentivize answers, and allow the most prevalent (need to know) RFC's bubble to the top of the list.

What are your thoughts on the format of this question?