Bitcoin Cash ABC added a new rolling checkpoint system in software version ABC 0.18.5, which was released on 21st November 2018. Essentially, the new mechanism finalizes a block once it has received 10 confirmations, which prevents large blockchain reorgs. Therefore even if an alternative chain has more proof of work, if it conflicts with a checkpoint, the node will not switch over to the most work chain.
This feature may have been added as a defence against potential attackers including from supporters of the rival Bitcoin Cash SV chain, who have indicated they may wish to attack Bitcoin Cash ABC.
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