It seems that the answer could be hidden in the "tiny" notes as part of the subtitle of the Class I Zone Group "c".

Thanks for trying, but I understood nothing.

Which NFPA number is that?

Good Luck,
Latexman

I'll grant you it's a bit odd, but some parties are better than others at getting the same layout than others.

In essence there is no difference between a dash, three dots (line ASME do) or a blank space. It's meaning is the same - i.e. there is no data or the item is regarded as not valid for design or operational purposes.

Adding letters isn't always a good move as it clutters up the page, but I agree spaces or voids are not the best.

So for me it's just whoever made the table did it that way and no one has imposed a set way so nothing gets changed....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

@Latexman
NFPA 497 2017 ed.

@LittleInch
still not clear

I agree it's inconsistent, but I think you're looking for an explanation / reason which doesn't exist other than lack of control by the authors.

Try writing to NFPA to ask them - maybe there is a reason which we don't know about, but it looks like a blank is "an absence of any statement", but "-" can also be used for the same thing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Sorry, no free access to that standard, but I did see a new Edition of NFPA 497 is due January 2024.

Good Luck,
Latexman

Are you sure about that standard number? I've looked and 497 2017 doesn't have a section 6....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Wouldn't Table 4.4.2 be in Section 4? In case he pulled from 2 different standards.

Good Luck,
Latexman

Source does not matter as such situation is common. Thank you all.

for info
table 4 from NFPA
table 6 from Carson&Mumford's Hazardous Chemicals Handbook 2nd ed.
I am a male

"I am a male" I fixed it.

Sorry, misinformation on your profile on another website.

Good Luck,
Latexman

I am on a couple of AMS (SAE) committees and they have updated their rules and actually follow them.
If an entry is blank or in table speak 'null value' they insert an ellipsis (...).
If it is a combination that is specifically not permitted, they will say so.
You have to remember that these documents are written by volunteers, with various backgrounds, and the original drafts often go back decades (>50years in many cases). and then they have been revised many times.
Lots of opportunity for differences in style to creep in.

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P.E. Metallurgy, consulting work welcomed

@EdStainless
I have no claims to volunteers, but every such doc has an editor, correct? Is an editor responsible to provide uniform design / style / logic whether editor is one person or a team where roles are devided. We are discussing not so overcomplicated issue, correct? Is editor a volunteer also? I think answer is negative as even if he/she/it got involved voluntarily he/she/it got most benefits from issueing such doc whether those are represented as power, reputation, promotion, contacts, money or whatever. Is an editor responsible to prevent such creeping in?

Again - we are discussing the issue (a) obvious to everyone taken such doc in hands, (b) not related to culture or background, and (c) correcting of that requires 1 hour. Are volunteers the reason? I think no.

My idea is this is System and I would appreciate if someone well known with the situation comments this and advises how a reader should react.

@Latexman
Don't spend your time
Even green cheese is ok

Either it's an error, or there isn't anything for them to put in the slot. There's nothing nefarious or magical here

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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There are editors, they are usually paid (or contracted) staff.
To be honest their focus with each revision is the changes.
Every now and then a spec gets the full overhaul, but those are a ton of work.
You would be surprised how often footnotes and other 'minor' things get misaligned in the process.
The part of spec creep is handled in the meetings and voting.
You can't downgrade a spec.
You can't have a spec favor a commercial interest.
And if you upgrade a spec it had better not negatively impact the price or availability.
This is why we need as many people as possible involved in the spec process.
AMS, SAE and ASTM are fairly easy to get involved with. ASME is a little more difficult.

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P.E. Metallurgy, consulting work welcomed

Pull together vast amounts of data, data that is not collected from identical testing, at best cannot be comprehensive. There are always voids.

To answer your question, put your money on the table and run the tests!