01Definition
There is no single global mempool. Each node applies its own relay policy, capacity and fee thresholds, so two honest nodes can temporarily hold different sets of unconfirmed transactions.
02Admission is not consensus
Mempool policy filters spam and manages resources, but it is not the same as block validity. A miner may include a consensus-valid transaction that some peers did not relay.
03Eviction and replacement
When memory limits are reached, nodes evict lower-fee transactions. Replace-by-fee policies allow compatible unconfirmed transactions to be replaced by versions paying a higher fee, supporting fee correction and other workflows.
⌖Reference points
01
Local to each node
02
Contains unconfirmed transactions
03
Relay policy differs from consensus
04
Capacity pressure raises fee thresholds