SAP ERP 6.0 (C_FSUTIL_60) Practice Test 1

In the attached image you can see a double-rate meter (Dev. 1), controlled by a ripple control receiver (ARCR). One of the registers in Dev. 1 has a relationship witha register in another meter (Dev. 2). Consumption in Dev. 2 is automatically deducted from consumption in the main register in Dev. 1 during the billing run.
What type of register relationship is described here?