Nodes running with the in-memory storage engine do not support transaction prepare and because of this, currently they cannot participate in cross-shard transactions.

On the chance that we start supporting that and in order to ensure that in-memory nodes and nodes running with writeConcernMajorityJournalDefault = false don't count towards the majority quorum for any w:majority writes done by the coordinator and participant, such writes should be done with j:true.