What end result do you want to achieve?

What does your employee handbook say about tobacco?

You could ask them for rent :) but only if you want to leave. Not sure they have the right to “apporopriate” your home like that though.

There may be a commitment to non-smoking office related to your employer's insurance premiums. We are living in unpleasantly interesting times, and HR may be trying to work out how to implement that in a work-from-home environment.

They said that because everybody is working from home, my home counts as my office while I'm working. - That seems like a stretch. Having said that, they can probably fire you without cause (laws depending), so don't give them cause. Whether or not this is a thing.

"Today, I got a formal notice from HR that I was receiving a citation for smoking in the office. " - what is the effect of a "formal citation" in your company?

It's considered unprofessional and provides an image the company does not want to be equated with. Smoking in your house they may not be able to "control". Smoking on a work-related call, that's something different. You're allowed to be nude in your house. You're not allowed to be nude on a conference call.

I think it is country specific. In France, you could go to court and win. Smoking is a constitutional right, and at my office people (including managers) are often going out of the building to smoke. With the covid crisis, they definitely smoke at home, including when videoconferencing. However, by so doing, you give a bad image of yourself.

Wow. Covid-19 has really made the world go mad.

@BasileStarynkevitch I'm sure that right only applies in breaks or after work, right? Smoking on the job certainly is no constitutional right any more than drinking alcohol or being nude is.

The right for a few minutes of break to smoke a cigarette on the workplace (outside of the building) has gone to court in France. I am not a smoker, but AFAIK the worker won.

Well, they have a right to a break and what they do in their break is their thing, so far that's perfectly understandable. What I mean is that there is no right to smoke on the job. The person in question here did not smoke in their break (or nobody would ever have found out) but while acting in a work function. There is no right to smoke while actively engaged in a work function.

"They said that because everybody is working from home, my home counts as my office while I'm working." I think most of your 'confusion' is taking this sentence too literally instead of trying to get the overall point they were trying to make.

You already got in trouble for smoking at home. Is your question whether they can legally fire you over it? Although would you really want to keep doing it despite their objections even if they can't fire you? Or is your question whether them reprimanding you is legal? But then the question is what direct consequence does a "citation" really have. Although really you got in trouble for smoking on a work call. If you were smoking outside of a work call, it might be a different story, but how would they even know about that (without constant monitoring, which may raise other concerns)?

It seems HR has nothing else to do and is trying to find "work" for themselves.

@user1936752 this is a formal discipline document hr sent you absolutely take it literally