What is a transaction lane in Kaspa's KIP21 protocol?

A lane is an ordered group of accepted transactions that all share the same subnetwork ID. In Kaspa's KIP21 design, every transaction carries a subnetwork ID — a label identifying which application or purpose it belongs to. KIP21 collects all accepted transactions with matching subnetwork IDs into one lane, preserving exactly the order they were accepted into the blockDAG. Future protocol upgrades may define additional ways to form lanes beyond subnetwork IDs. This matters because lanes give different applications running on Kaspa their own sequenced record of transactions, laying the groundwork for app-specific ordering guarantees.