Assembly: Bass.Net (in Bass.Net.dll) Version: 188.8.131.52
Property ValueType: Int64
It will be incremented with every short-message created in the system. So it might not be linear when using multiple devices in parallel, but it will for sure be incremental for each device, meaning each message ID generated on a device will be greater as the the previous message.
But note, that the counter will be wrapped to 0 when it reaches a value of 9,223,372,036,854,775,807.