Does anybody know what the difference between the two files?
And when will the CISCO-ENVMON-CAPABILITY.my be used?
I found below description but could not exactly understand what it meant.
Agent-capabilities for CISCO-ENVMON-MIB.
The MIB module to describe the status of the Environmental Monitor on
those devices which support one.
MIB locater uses CISCO-ENVMON-MIB.my when searched by the supported image or the device/IOS.
The CAPABILITY file does not dictate which devices support the MIB. It lists what deviations exist from the MIB. For example, if some platforms implement the CISCO-ENVMON-MIB, but do not implement a certain object in the MIB, then that should be listed in the CAPABILITY file. It is not necessarily true that just because a device supports a given MIB that it will support all the objects in that MIB.
The MIB Locator is still the authoritative source as to what devices and what versions of IOS support what MIB modules. The CAPABILITY files further refine that (though even those are not perfect).
It does appear that the 2821 supports the ciscoEnvMonRedundantSupplyNotification in 12.4(21).