What is difference between Action on Service Down Reselect and LB::reselect

Action on Service Down Reselect is a pool setting that says to select a new pool member when the current pool member gets marked down. The LB::reselect command is invoked in an iRule and "is used to advance to the next available node in a pool, either using the load balancing settings of that pool, or by specifying a member explicitly." This means the command can be used whenever and doesn't require a pool member failure to be invoked.

https://devcentral.f5.com/wiki/iRules.LB__reselect.ashx