There is likely no real standard to behavior in the world of the Japanese martial ways known as budo (武道). There may also be differences between arts, schools, regions, and so on. Much like the rest of the world human behavior doesn't always follow the same standards.
I've heard it said that the etiquette of this world comes from Ogasawara-ryu (https://www.youtube.com/watch?v=TNZM0ALGIiI).
The following are various behavioral things I've either been corrected about or have seen others corrected for at iaido seminars.
In no particular order:
Do not cross your arms. I'm not entirely sure why, as I saw senior instructors crossing their arms later in the seminar. A listener wrote to tell me that: "Crossing the arms can come across as insolent, disrespectful, and unimpressed, particularly if done by a junior person. Some seniors don’t think this applies to them."
Do not wear a watch. I was told it shows that I'm not serious about training. I can understand why this is problem if we are touching each other. It can become a hazard in other arts where people are grabbing each other. I'm not entirely sure there's a practical reason for this in iaido. A listener wrote: "I was told wearing a watch might catch on your clothing and/or get in the way of adequately using your wrists, very important in iai."
Do not lean against anything. This was also apparently an indicator of how I wasn't serious about training. A listener wrote: "Leaning against something gives a similar impression to crossing your arms."
Listener contribution: Do not lean on one’s weapon. "This shows a lack of respect towards it." Note: See the link to Ellis Amdur's post at Kogen Budo about etiquette and weapons below. This falls into the same category of following the rules when handling firearms on the range.
Listener contribution: Sit in seiza. "When sitting to watch an embu or listen to an instructor, at least start in seiza as this is a more respectful way of sitting. If it gets painful you can change to a more comfortable position afterwards by making a small bow."
Listener contribution: Avoid showing the soles of your feet. "Again, this is incredibly rude and disrespectful in a number of cultures, including Japanese."
Do not refer to instructors by their first name. This is definitely a cultural and linguistic thing in Japan. It doesn't matter whether it's budo or somewhere else. There is an inherent built in respect for instructors and teachers in Japanese. It makes no sense linguistically to refer to a teacher as [First Name]-sensei as that would be far too familiar. When on the mats refer to instructors as [Last Name]-sensei. A listener wrote: "This is a really interesting point. It’s obviously very true here in Japan but, in Europe, pretty much everyone uses the person’s first name followed by “Sensei” (if the instructor is non-Japanese) which, to me, makes much more sense than using their family name. No one took it as being too familiar/casual/disrespectful anyway, good to know!"
Learn some Japanese. I am already functional in Japanese. Not great, but enough to get the gist of what is being talked about while missing the details. Much of the seminar was in Japanese. Once in while someone would do a little translation, but a lot of the time there wasn't. At the very least study up on your iaido specific Japanese vocabulary. That will help you follow along. Here are a few to start with: https://www.auskf.org/department/iaido/iaido-study-guide This post may help too: https://walkingtengu.wixsite.com/tengu/post/classical-martial-arts-ideas
Above all, pay close attention to what everyone else is doing. There may be differences from what you and I experience. The best thing to do in any situation is keep your pulse on what is going on in the room you are in right now.
If you know of any other etiquette faux pas a newcomer to the iaido seminar world should know, please reach out and let me know so I can add it to the list.
Additional Resources
Canadian Iaido Association - Seminar Rules & Etiquette
Doshikai - The Etiquette of Iaido
artofiaido.com - Etiquette
Kamusokai - Etiquette
General Etiquette For Any Seminar
"The Real Importance of Reishiki in Koryu"