The values set for the threshold PVs (listed in here) are tracked by an MPS runtime database. Any changes to thresholds must go through the MpsManager server, which records the changes in the runtime database and then changes the PVs.
When an IOC reboots it automatically issues a request to the MpsManager to restore the threshold PVs. The request happens after iocInit().
Once thresholds are set by the MpsManager the $(PREFIX_BASE):MPS_EN is set to enable - this PV must be enabled in order to start MPS in the application IOC. It must be enabled only after the thresholds have been properly set by MpsManager.