This article describes the power zoning in MX960

Both normal-capacity and high-capacity MX960 chassis with DC power supplies; MX960 chassis with high-capacity AC power supplies; MX960 high-capacity second-generation AC power supplies; and MX960 high-voltage second-generation universal power supplies are zoned.

Zoning means that certain components are powered by specific power supplies

In the DC power configuration, the router contains either two or four DC power supplies, located at the lower rear of the chassis in slots PEM0 through PEM3 (left to right). You can upgrade your DC power system from two to four power supplies.

Zone 0 - The DC power supplies in slots PEM0 and PEM2 provide power to the lower fan tray, FPC slots 6 through 11, and SCB slots 1 and 2.

Zone 1 - The DC power supplies in slots PEM1 and PEM3 provide power to the upper fan tray, FPC slots 0 through 5, and SCB slot 0.

In high-capacity AC power supply configurations, there are two zones that provide power to specific components in the MX system. No current sharing between power supplies is needed with the high-capacity system because the redundancy changes from 3+1 per system to 1+1 per zone.

Zone 0 - The MX960 AC high-capacity power supplies slots PEM0 and PEM2 provide power to the lower fan tray, FPC slots 6 through 11, and SCB slots 1 and 2.

Zone 1 - The MX960 AC high-capacity power supplies in slots PEM1 and PEM3 provide power to the upper fan tray, FPC slots 0 through 5, and SCB slot 0.

Please refer below technical publication for more details :

https://www.juniper.net/documentation/us/en/hardware/mx960/topics/concept/calculating-power-requirements-mx960.html