Concurrent Limit Definition in CRM REST API

I just had a one off issue in my CRM REST API integration

`429 | TOO_MANY_REQUESTS | Many requests fired in concurrent than the allowed limit.`

I understand the issue, the concurrency limit is explained in the documentation:

These concurrency limits specify the maximum number of API calls that can be simultaneously active at a given point of time per org per app.

Similarly it also described in this older announcement:
Therefore, the rate limits, or the 'concurrency limits' as it shall be termed henceforth, will now specify the maximum number of API calls that can be simultaneously active at a given point of time, for a user per app.

The question is how exactly is defined simultaneously? Is it an open connection, i. e. number of requests that have not resolved yet?

Let' say I have the limit 25 in ultimate so each time I make a REST call there must 24 or less requests that have not resolved yet in that point of time; otherwise that new REST calls throws 429?

I appreciate this potentially a silly question and I am guessing there is a straight forward answer.