When a transaction is presented to a node as part of a flow it may need to be checked. Simply sending you a message saying that I am paying you £1000 is only useful if you are sure I own the money I’m using to pay you. Checking transaction validity is the responsibility of the ResolveTransactions flow. This flow performs a breadth-first search over the transaction graph, downloading any missing transactions into local storage and validating them. The search bottoms out at the issuance transactions. A transaction is not considered valid if any of its transitive dependencies are invalid. It is required that a node be able to present the entire dependency graph for a transaction it is asking another node to accep