Code and data structures from PLC through SCADA to DB that hold critical information that simply cannot be allowed to fall into the bit bucket on the floor.
Usually configured to hold in the PLC some small multiple of the maximum connection downtime that can be predicted for the overall system. Data is delivered, usually many pieces in parallel, through SCADA to the high-reliability database, where each piece of data gets a distinct handshake to the PLC after its insert transaction succeeds.
The PLC would be programmed to gracefully shut the process down if it hits some capacity threshold in the buffer. The data transfer bandwidth in normal conditions would have to be somewhat faster than average data production in the process, to have the ability to "catch up" after disruptions.
The "ring" in ring buffer indicates a technique in the PLC that avoids moving data in the buffer, but instead adjusting subscripts for head and tail locations.