The Ardor system are defined relative to a whole unit of the holding being transactioned. The parent chain ARDOR has a currency ARDR used to quantify valu of forging in the system. Like all currencies, ARDR circulates in the system, moving from one user to another by transfers. Also, a small fee is required for every transaction, including those in which no ARDR is transfered, such as simple messages when using a child chain. This fee goes to the owner of the node that forges (generates) the new block containing the transaction that is accepted onto the blockchain, and the bundlers get the child chain fees and pays the ARDR fees to the forgers.