"rhsyns" - what does it stand for?

To be clear you are saying the whole thread should have been about

https://whatsonknowledgebase.mediagenix.tv/2023r2/index.htm?rhcsh=1#t=Trailer%2FRatings%2FRatings_Workflow.htm&rhsearch=%22ratings%22&rhhlterm=%22ratings%22&rhsyns=%20. Correct?

If so, my guess would be that the user has accessed the topic through search rather than the TOC, cross topic linking and so on.

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It looks like a Classic thing then, not New Ui.

Yes I did. I tested in 2022.1. You haven't said which version you are using and it looks like @Amebr tested in a Classic version, based on the output path including SSL. 

I have tested both Frameless and Responsive templates (skins).

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

Searching for synonym gives me a string ending as below.

00_Test_Projects/Outputs/Frameless/Second_Topic.htm?rhsearch=pink&rhhlterm=red

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

I think it is to do with synonyms although this is my own opinion and not something I remember Adobe ever addressing. I think "rhsyns=%20" by itself means the accessed topic was ffound using the search term. Whereas "rhsysn=%20xxxx" indicates thes topic was found using a synonym of the search term, with the synonym listed after the %20 part.

The following was done with Classic Responsive HTML5 and Azure skin, but you should be able to test the same setup in New UI as well.

For example I set up a topic that contained the word 'swede' and set up a synonym pair of 'swede-rutabaga'. When I searched for swede I got the following url after clicking on a search result:

!SSL!/Responsive_HTML5/index.htm#t=temp.htm&rhsearch=swede&rhhlterm=swede&rhsyns=%20

When I searched for rutabaga I got this link after clicking on a search result:

!SSL!/Responsive_HTML5/index.htm#t=temp.htm&rhsearch=rutabaga&rhhlterm=rutabaga&rhsyns=%20swed

(why the synonym is missing the last letter I'm not sure - it's how it appears in the URL in this example, not a typo)

Do note that the only term that appears after the rhsyns property is the first term in a pair of synonyms. So if you delete 'turnip' from the topic, replace it with 'rutabaga' and then search for 'rutabaga', you will still get &rhsyns=%20swed  at the end of the url. 

My 100% guess is that this has something to do with synonyms in the search, based on the tag name. I have tried to use the synonyms (to no avail) to see if it changes the URL somehow. Feature seems broken (or I am totally inept). 

That looks like a search - not a CSH call. Not sure what the "rhsyns" means but I suspect it's something that is being used in the search algorithm. You could always fire off an inquiry to the RH folks - see https://helpx.adobe.com/contact/enterprise-support.other.html#robohelp for your Adobe Support options. I'd recommend using the tcssup@adobe.com e-mail address as it reaches a team dedicated to Technical Communication Suite products including RoboHelp.