Hello sales2161
According tothis Cisco documentation, the difference is that the state and reachability options have to do with the OverThreshold return code. As stated in the documentation:
Two aspects of an IP SLAs operation can be tracked: state and reachability. The difference between these aspects relates to the acceptance of the OverThreshold return code. Table 79 shows the state and reachability aspects of IP SLAs operations that can be tracked.
Now what does this all mean? Well, practically speaking, when you configure a threshold, the reachability option only takes into account if you can reach the destination IP address with the Echo IP SLA operation. The state option also takes into account the Round Trip Time (RTT) of the Echo IP SLA operation.
I hope this has been helpful!
Laz