The ACX is primarily an MPLS edge and aggregation node. This does not support virtual chassis which is what the MX series devices use for clustering. Chassis cluster is SRX only.
There are some High Availability options like VRRP and routing protocols offered on the ACX. But as a lower cost edge node there are limited resources and features enabled on the platform. There is good support for OAM and the full redundancy feature sets in MPLS, l2circuits and VPN services.