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Primary terms were introduced as part of the sequence-number effort (#10708) and added in ES 5.0. Subsequent work introduced the replication tracker which lets the primary own its replication group (#25692) to coordinate recovery and replication. The replication tracker explicitly exposes whether it is operating in primary mode or replica mode, independent of the ShardRouting object that's associated with a shard. During a primary relocation, for example, the primary mode is transferred between the primary relocation source and the primary relocation target. After transferring this so-called primary context, the old primary becomes a replication target and the new primary the replication source, reflected in the replication tracker on both nodes. With the most recent PR in this area (#32442), we finally have a clean transition between a shard that's operating as a primary and issuing sequence numbers and a shard that's serving as a replication target. The transition from one state to the other is enforced through the operation-permit system, where we block permit acquisition during such changes and perform the transition under this operation block, ensuring that there are no operations in progress while the transition is being performed. This finally allows us to turn the best-effort checks that were put in place to prevent shards from being used in the wrong way (i.e. primary as replica, or replica as primary) into hard assertions, making it easier to catch any bugs in this area.