The POLO charge is the outpayment that BT Wholesale pays the terminating Communications Provider for terminating a call on their network. Typically this is calculated as the ROLO charge minus the TWIX charge but not always, e.g. in the case of freephone numbers.
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 geographic number. Garytel pays BT Wholesale the ROLO charge. BT Wholesale then route 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 the POLO charge.