I hope you are refering to the below flag, which refers to the client as Standby mode AP, when you have a HA setup.
#show ap database
AP Database
-----------
Name Group AP Type IP Address Status Flags Switch IP Standby IP
---- ----- ------- ---------- ------ ----- --------- ----------
AP205 ha1 205 10.163.163.32 Up 31m:3s S <<<<<<<<< 10.163.160.97 10.163.160.98
AP215 ha2 215 10.163.163.31 Up 2h:21m:13s 10.163.160.98 10.163.160.97
Flags: U = Unprovisioned; N = Duplicate name; G = No such group; L = Unlicensed
I = Inactive; D = Dirty or no config; E = Regulatory Domain Mismatch
X = Maintenance Mode; P = PPPoE AP; B = Built-in AP; s = LACP striping
R = Remote AP; R- = Remote AP requires Auth; C = Cellular RAP;
c = CERT-based RAP; 1 = 802.1x authenticated AP; 2 = Using IKE version 2
u = Custom-Cert RAP; S = Standby-mode AP <<<<<<<<<<; J = USB cert at AP
i = Indoor; o = Outdoor
M = Mesh node; Y = Mesh Recovery
Below command give HA AP table.
#show ha ap table
HA AP Table
-----------
AP IP-Address MAC-Address AP-flags HA-flags
-- ---------- ----------- -------- --------
AP205 10.163.163.32 9c:1c:12:c7:e1:24 SLU <<<<<<<<<<<<<<
AP215 10.163.163.31 24:de:c6:cf:65:02 LU
Total Num APs::2
Active APs::1
Standby APs::1
AP Flags: R=RAP; S=Standby <<<<<<<<<<<; s=Bridge Split VAP L=Licensed; M=Mesh, U=Up
You can find more information about HA in the Support Knowledge base site and the User guide.
http://community.arubanetworks.com/t5/Controller-Based-WLANs/How-does-high-availability-fast-failover-feature-work-and-how-to/ta-p/177468