What are M:N transactions in Kaspa, and why do mining pools need them?

An M:N transaction in Kaspa is one that combines multiple inputs with multiple outputs in a single transaction — for example, several coinbase rewards coming in and dozens of participant payouts going out. Mining pools need M:N transactions because Kaspa's KIP-9 rules impose mass penalties when outputs outnumber inputs; structuring payouts as M:N keeps the ratio balanced and avoids those extra fees. KIP-10 threshold scripts make M:N pool transactions practical by allowing the pool to use each participant's KIP-10 compatible P2SH address UTXO as an input, giving the pool enough inputs to match its many payout outputs. For participants, the result is that the pool can pay many people at once efficiently, keeping fees low and payouts frequent.