This defines if the settings underneath have an effect or not

Search Engine will not allocate shards to nodes that have less diskpace available than this value. This setting has no effect on the primary shards of newly-created indices but will prevent their replicas from being allocated.

Search Engine will attempt to relocate shards away from a node whose available disk space is less than this value. This setting affects the allocation of all shards, whether previously allocated or not.

Search Engine will put indices in read-only mode if available disk space is less than this value. This setting is a last resort to prevent nodes from running out of disk space. The index block is automatically released when the disk utilization falls below the high watermark.