some further documentation?

[jurgenweber]

what is this?

-f FAILURE_DOMAIN, --failure-domain=FAILURE_DOMAIN
A comma-separated list of ElasticSearch attributes
that make up your cluster's failure domain. This
should be the same list of attributes that
ElasticSearch's location-aware shard allocator has
been configured with. If this option is supplied,
additional checks are carried out to ensure that
primary and replica shards are not stored in the same
failure domain.

Primarily what is the failure domain? and searching for it does not yield any results so just want ot know what I should be setting this to, if anything?

[olorin]

Failure domain is something like rack_id or zone, as described here.

If you don't know what it is for your cluster you can ignore the option in the check.