Abstract
By default Iroha works with single-signature transactions which are accepted by the peers signed only by one signatory. However in some cases it is not enough.
Example
Let's take an example of selling good from Seller to Buyer with independent Arbiter involved . Buyer sends XORs Transfer Iroha Special Instruction needed to cover the price of the good.
Seller should send a physical good to the Buyer to complete XORs Transfer. Multisignature transaction could be use to make this process safe.
M-of-N
If M is the required number of signatures and N the total number of available signatures so in this example we need to set `M=2` and `N=3` (restricted by Signatures of Seller, Buyer and Arbiter).
In case of an "easy" deal Buyer and Seller both add their signatures to the transaction letting Iroha to process Transfer Instruction. If one of the parties decided to not sign the transaction, Arbiter can decide to add or not to add it's signature.
If Arbiter will try to Transfer XORs to itself, it will fail because one signature is not enough in this situation.
Introduction
Conditional multisignature
From the whitepaper: https://github.com/hyperledger/iroha/blob/iroha2-dev/iroha_2_whitepaper.md#conditional-multisignature
Let's consider a situation where a bank wants to allow either 2 tellers or 1 manager to sign off on any transfer transaction over $500 and under $1000. In this case, the condition will be: Condition.asset("usd@nbc").qty(500).comparison(">").qty(1000).comparison("<") and the signatory_sets for the tellers and manager will be OR unioned, so that either the m-of-n signatues from the tellers or the single signature from the manager will be acceptable for transaction signing.