This value is in OplogApplier::Options and it's a parameter to SyncTail::syncApply and all the lambdas in kOpsMap. According to the comment in OplogApplier, "For replication recovery only. During replication rollback, this is used to keep track of the stable timestamp from which we replay the oplog." After the Oplog Durability project, however, this parameter is unused.

As pavithra.vetriselvan says, "This is no longer needed after the oplog durability project because we do not need to compare the commitTimestamp to the stableTimestamp. This was originally used to tell whether prepared operations were already reflected in the data."