In general you can use Data Model Discovery tool to peruse the definitions and uses of specific real-time data fields such as this one. However for this specific field Data Model Discovery tools does not seem to have an explanation you're looking for. If Data Model Discovery tool does not answer your question, the next resource for content explanation questions like this one is Refinitiv Helpdesk, which you can reach by either calling the Helpdesk number in your country or by using Contact Us capability in your Eikon application.
Please note that the use of specific real-time field may vary by asset class and geography. It would be helpful if you define your universe of instruments or provide some example RICs when contacting the Helpdesk.

@Alex Putkov. Thanks for the tips, however, I did use the Eikon helpdesk and they told me that I could find the information out here since they do not know about the data fields in the API.

@DavidBen.Anderson
I can see that the Helpdesk initially got confused, since the FID you're enquiring about does not exist on Refinitiv real-time datafeed for the RIC COUK.L you included as an example, and since you're not retrieving the data directly from any Refinitiv products, but rather from your own SQL database. I also see that once you clarified your question in communication with the Helpdesk, they responded with:

"Hi David, I apologize for the delay in response. The description of the FID 1964 [STLVAL2_7] is - the value of the Nth settlement item the latest but one. I confirm that this FID is not sourced from LSE, thus explaining why you do not get the queried data for example RIC <COUK.L>. Let me know if you need further clarification."
The case is still open pending either an acknowledgment that you're fine with the response or further communication from you.