The TWIX charge is the charge that BT Wholesale take from the originating Communications Provider for routing calls over the BT Wholesale network. Typically this charge is paid by the originating provider and the terminating provider receives what is left of the ROLO charge, known as POLO.
Assume, for the purposes of this example, Garytel and gr0mtel are both BT Wholesale customers and they are not connected to each other's network. A customer of Garytel makes a call to a gr0mtel customer's geographic number. Garytel pays BT Wholesale the ROLO charge. BT Wholesale then routes the call to gr0mtel's network, taking the TWIX charge as payment for doing so. BT Wholesale then pays gr0mtel what's left of the ROLO charge after taking TWIX, and this is known as the POLO charge.