The difference is the regulatory domains of these APs, as can be seen in the data sheet.
*JW811A Aruba Instant IAP-315 (RW) 802.11n/ac Dual 2x2:2/4x4:4 MU-MIMO Radio Integrated Antenna AP
*JW813A Aruba Instant IAP-315 (US) 802.11n/ac Dual 2x2:2/4x4:4 MU-MIMO Radio Integrated Antenna AP
So the hardware and software are the same, but they are restricted in terms of the regulatory domain.