The repThreshold (bidding.relocationThreshold) - also called relocationThreshold in the SNMP MIB- setting is the allowed percentage utilization difference between the volumes before CAStor will proactively load balance and send a new replica to the under-utilized volume.
Here's the text from the Admin Guide configuration section:
Replication threshold. The value of this parameter is an integer between 0 and 100, representing the allowed difference between volume percent utilizations. Lower values ensure better load balancing and higher throughput. Higher values minimize data movement at the expense of lower maximum throughput.
The default is 5%, so if the existing volumes are 97% full and you add a new empty server, CAStor will fill the new disks to 92% full with replicas of existing objects. If you are trying to preserve space on your newly added disks for new streams, we suggest setting relocationThreshold through SNMP to 50 so that no more than 47% of the newly added space will be immediately filled with replicas of existing objects, preserving available space for new writes while defrag works on the full volumes.