If you run the system mode as transparent, you'll need to cluster the inside interface.  In running as transparent mode, your real servers have to use Array's inside IP as the default gateway.  Since only one default gateway can be set on the real server, you would use this inside VIP as the default gateway of your real server. 

Example:
ip address "outside" 10.1.160.20 255.255.0.0
ip address "inside" 172.168.1.20 255.255.255.0
cluster virtual ifname "inside" 1
cluster virtual vip "inside" 172.168.1.5
cluster virtual auth "inside" 1 0
cluster virtual interval "inside" 1 5
cluster virtual preempt "inside" 1 1
cluster virtual priority "inside" 1 100 Primary
cluster virtual priority "inside" 1 100 TMX1
cluster virtual priority "inside" 1 50 TMX2

In this example, you will use 172.168.1.5 as default gateway of your real server.  This way the cluster knows how to manage traffic between the Array and the real server.