ACP is in effect a daisy chain between all entities in your HA storage system.
You come out of one NetApp FAS controller and go into a controller on the disk shelf and then through the same controllers on that stack; then daisy can the stacks together and before you know it you are back in the other controller.
The cabling diagram that ships with any new system or new shelf can help with this.
https://library.netapp.com/ecm/ecm_get_file/ECMM1280392