This is a quality control flag.
M = Merged data, a new post-processing scheme being implemented at IRIS DMC to present clean data traces from multiple pieces of data. Meant for suturing various qualities of data into a single, more continuous time series, to make analysis easier and eliminate time tears in data.
Q = Data tagged as Quality Controlled by the network operator
D = Data of indeterminate quality—this is a legacy tag for older data. This was the only designator until SEED v2.4
R = Raw Data as a result of real-time telemetry to the DMC
For more details about the markers and how you can customize your data requests, please view this DMC newsletter article.