IOB (Inter-Operator Billing) refers to the process of billing and settlement between different mobile network operators (MNOs). This is important when a subscriber from one operator's network (the Home Public Land Mobile Network or HPLMN), roams onto another operator's network (the Visited Public Land Mobile Network or VPLMN).

When a subscriber roams onto the VPLMN, the VPLMN tracks and records all the usage made by the subscriber, such as voice calls, SMS, data usage, etc.

The VPLMN periodically sends billing records to the subscriber's HPLMN. This includes the usage and charges incurred by the subscriber on the VPLMN. This process is known as Inter-Operator Billing (IOB).

Taking into consideraton that the HPLMN and VPLMN have a pre-negotiated wholesale billing rate for different services. Which is agreed upon in the roaming agreement between the two operators.

Accordingly, the HPLMN then incorporates the roaming usage charges from the VPLMN into the subscriber's monthly bill.