This articles covers some of the issues that come up when using trustlines and offers potential methods of resolution.
N.B. Issued Currencies are commonly referred to as tokens or IOU's.
Those terms may be considered interchangeable.
PathsIn the XRP Ledger, paths define a way for issued currency payments to flow through intermediary steps on their way from sender to receiver.
Paths enable cross-currency payments by connecting sender and receiver through orders in the XRP Ledger's decentralized exchange.
Finding paths is a very challenging problem that changes slightly every few seconds as new ledgers are validated, so
[the XRP ledger protocol]is not designed to find the absolute best path.
Still, [multiple] possible paths [can be found] and the cost of delivering a particular amount estimated.
Trustlines are one type of path. They define connections between accounts and set parameters on what and how much of a particular Issued Currency (AKA Token/IOU's) the Account Holder is willing to trade.