The Coordinator - PoA Consensus
The Coordinator is a temporary feature running under the Chrysalis and Stardust protocol versions and will be removed with the upcoming IOTA 2.0 update.
The Coordinator is a node controlled by the IOTA Foundation that sends signed blocks called milestones that nodes trust and use to confirm blocks and reach consensus. Blocks which are directly or indirectly referenced by a milestone block are automatically deemed confirmed, though whether they mutate the ledger depends on the White-Flag consensus approach. Before the White-Flag consensus was introduced, nodes would instead compute mutations via cumulative weight calculations.
Even though the Coordinator acts as a centralized confirmation device or finality gadget, it is not able to move or alter the ledger in any other way than given by the user issued transactions in the DAG. Therefore, the Coordinator can only censor transactions (which would be more visible than in traditional blockchains) but not actively mint or remove tokens from the ledger.
Nodes within an IOTA network running under PoA via the Coordinator have its public keys saved in the configuration and hence validate whether milestones are really issued by the Coordinator. Milestone are regularly issued by the Coordinator and determine the confirmation latency of the network. One can think of as milestones as the equivalent of block headers in more traditional blockchains, although in the case of IOTA, the block header confirms a subgraph within the DAG.
The Coordinator