You're right. It's documented in the older APIv1 documentation at https://www.logicmonitor.com/support/rest-api-developers-guide/v1/devices/about-the-device-resource . The old documentation is still important even for v2 context.
sdtStatus: This parameter has three values separated by ‘-‘. The first value indicates whether there is an active SDT for a group the device belongs to, the second value indicates whether there is an active SDT for the device itself, and the third value indicates whether there is an active SDT for a datasource or instance on the device