EIGRP measures reliability as an estimated metric of an interfaces ratio between successfully received and all received frames. the ratio is presented as a fraction in the ‘show interface’ output. A reliability of 255 means 100% reliability. Reliability is dynamically updated by the IOS.
When calculating the composite metric to a destination EIGRP takes the minimal reliability into account. It’s done by comparing the reliability advertised by a neighbor to the interfaces reliability facing that neighbor and takes the lower value between the two.
EIGRP does not send updates when reliability of an interface changes, the metric is simply a snapshot of the then current reliability when the route was last advertised.
This metric is optionally used in the composite metric calculation and is pretty much just an older deprecated metric with no particular usability today.