It is legal mandate

Why does the Fed believe that inflation is going to rise? That’s what the econoblogosphere has, of late, been trying to figure out. 

Fed officials know the inflation mandate is useless since the selected index is arbitrary. 

The implicit deflator is two quarters old, unusable. So the official inflation mandate is simply BS.  

The Fed members are really worried about price distortion, friction causing bubbles, The Fed cannot say this, way too many economists get confused because this is really distribution skew.  Inflation was a generic term meaning 'to get bigger', implying distribution mean, or the implicit.

Better way to think mandate:

The best mandate is that any currency issuer is hedged unless its process is the fastest path. The fastest path is  the  imbalance between deposits and loans, the currency tradebook, which  has all possible information.  Then the distribution is constructed, as specified by the congested buy and sell queues.