What is the counter(no sunbrelations) in success events type setting?

First of all I think this documentation is obsolete https://experienceleague.adobe.com/docs/analytics/analyze/reports-analytics/customize-reports/subrelations.html?lang=en . The reason is that it relates to Reports tab so the old reporting feature of Adobe Analytics (inherited from SiteCatalyst).

Nowadays you should use Analysis Workspace instead of Reports. Most legacy limitations and specific configurations do not apply anymore so the setting Counter (No subrelations) does not apply I believe...

You should check this documentation for events and the different types: https://experienceleague.adobe.com/docs/analytics/components/metrics/custom-events.html?lang=en

and this one https://experienceleague.adobe.com/docs/analytics/implementation/vars/page-vars/events/events-overview.html?lang=en

So you are correct when you say that counter events can increment a value by more than one but the value needs to be an Integer. By default if you do not specify a value for the counter event it will be incremented by one, but in some cases you would want to increment it by more than one:

// Count event1 ten times
s.events = "event1=10";

// Count event1 twice and event2 once
s.events = "event1=2,event2";

Now the difference between counter and numeric:

- counter only accept Integer values (so without decimals) where numeric accept float values (with decimals) and also Integer .

Now the difference between numeric and currency type. Both accept float values and integer value but unlike numeric type, currency will also convert the value provided once received based on the report suite currency type. So if in your code you specify the value in dollars and your report suite is configured to use British pounds then it will convert your dollar value to the conversion rate of British pound on that day.

Hope this helps