no

Ad services will reveal what is required. If they don't say, don't worry about it.

Thank you - that's reassuring.

I have at last received some sort of reasonable response from CJ, which almost sounds as though they have no idea what an ads.txt file is, and that it is simply a requirement of GA .
Is this so?

If all you are running for advertising is GA, then that's probably correct.

Ads.txt has been developed to list ad networks which are authorized to propose your ad slots to their advertisers, within the context of bidding system. So in the case of CJ (or any other affiliate networks), no one is selling your ad slots, it's you who decide which ads to show in your slots.

Yep, it's how you mark your own site, to prevent programmatic fraud.
You post your ads.txt file on your website (like you post your robots.txt file), so the world can see who you say are authorized ad sellers of your inventory.

CJ affiliates don't buy your ads space on your site, nothing to do with them.

More explanation here:
[iabtechlab.com...]

See this paragraph there:
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When a brand advertiser buys media programmatically, they rely on the fact that the URLs they purchase were legitimately sold by those publishers. The problem is, there is currently no way for a buyer to confirm who is responsible for selling those impressions across exchanges, and there are many different scenarios when the URL passed may not be an accurate representation of what the impression actually is or who is selling it. While every impression already includes publisher information from the OpenRTB protocol, including the page URL and Publisher.ID, there is no record or information confirming who owns each Publisher.ID, nor any way to confirm the validity of the information sent in the RTB bid request, leaving the door open to counterfeit inventory.
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@RhinoFish ... thanks ... nice short and concise explanation of ad.txt.

Still serving 404's ... but that's a different story. :)