What Is the Difference between a Node’s Mempool and the Global Set of Unconfirmed Transactions?

A node's mempool is a local, private collection of unconfirmed transactions that that specific node has received and validated. The global set of unconfirmed transactions, however, is a theoretical concept representing all valid, unconfirmed transactions currently floating across the entire network.

Due to differing relay policies, network latency, and memory limits, no two nodes have an identical mempool. The global set is the union of all individual mempools, though it is never perfectly known or observed by any single entity.