What it holds
Two things: who they are — a person or a company, with the identifying details the rail requires — and where money reaches them, which is either a bank account or a wallet address. Which of those two it carries decides the rail, and the counterparty is where that name lives: a bank account is saved asACH or WIRE, a wallet as WALLET. Everything downstream — what a payment costs and how fast it lands — follows from that one choice. See Rail.
Why it is saved rather than sent
Paying the same supplier ten times should not mean sending their bank details ten times. Each send is a chance to mistype an account number, and a wrong number on a payment is not always recoverable. Saving the counterparty once moves that risk to a single moment you can check.What it is called elsewhere
In the API reference a counterparty is a recipient. The word is the same idea throughout — see The recipient object.A counterparty is not a sub-client. A sub-client is your customer, whose money it is. A counterparty is who that money is going to, and Kira does not verify them the way it verifies a sub-client.