OsmoBTS doesn't use the "Diagnostics" part of the RSL Cause IE
When we report an error cause as part of any CHAN ACT NACK, ERROR REPORT, MODIFY NACK, etc. in RSL, there's an option to report diagnostics along with the error cause.
The table in 3GPP TS 48.058 9.3.26 indicates the meaning of the diagnostics in context of the specific cause value. Sometimes it means "Channel Number", some times "Message Discriminator", sometimes "Message Type", sometimes "Element Identifier".