I was unable to find a good documentation/explanation as to what severity indicates in nodetool gossipinfo. was looking for a detailed explanation but could not find a suitable one.
The severity is a value added to the latency in the dynamic snitch to determine which replica a coordinator will send the read's DATA and DIGEST requests to.
Its value would depend on the IO used in compaction and also it would try to read /proc/stat (same as the iostat utility) to get actual disk statistics as its weight. In post 3.10 versions of cassandra this is removed in https://issues.apache.org/jira/browse/CASSANDRA-11738. In pervious versions you can disable it by setting -Dcassandra.ignore_dynamic_snitch_severity in jvm options. The issue is that it weighting the io use the same as the latency. So if a node is GC thrashing and not doing much IO because of it, it could end up being treated as the target of most reads even though its the worst possible node to send requests to.
Now you can still use JMX to set the value still (to 100) if you want to exclude it from being used for reads. A example use case is using nodetool disablebinary so application wont query it directly, then setting the severity to 100. That node would then only be queried by cluster if theres a CL.ALL request or a read repair. Its a way to take a node "offline" for maintenance from a read perspective but still allow it to get mutations so it doesn't fall behind.