In 1978, the first American satellite was launched into an earth orbit. Since then, new GNSS have been sent to the orbit, like GLONASS (russian), GALILEO (european) and BEIDOU (chinese). Today, most of professional GNSS receivers are able to track these different GNSS and most of them even to generate observation data. These observables can be used from other GNSS receivers to determine highly accurate positioning data and are presented in proprietary formats, so compatibility between different vendors can be achieved. There are already standardized GPS and GLONASS RTCM-3 messages which are used for base-rover RTK operation, but they cannot efficiently be extended to other GNSS and other signals (e.g. L5). These legacy messages were limited to L1 and L2 bands and to only one signal per band.
MSM is a new universal real time correction data format, that contains all GNSS observation data in generic form. This message type got introduced as the current key RTCM-3 concept by defining the RTCM v3.2 standard. The MSM standardization process started in 2012 by creating a dedicated MSM Working Group. Most of the professional GNSS vendors actively participated. That's why the MSM standard is highly compatible with all the different GNSS receivers.