This is a running count of MAC addresses that have been updated by the device, for example when the next-hop port for an already-learned MAC address changes. Per the module's revision history, this counter was added specifically to track port-move events for existing MAC entries, distinct from brand-new learns or full removals. A laptop that moves from one switch port to another without ever disconnecting long enough to age out would increment jnxMacAddressesUpdated rather than jnxMacAddressesLearnt.