Help understand UART diagram
In all probability:


Rxd is the state of the received serial data pin.
rsr is the receive shift register.
dcnt is probably a bit count.
b looks like the state of the Rxd pin sampled at the center of a bit time, but not having the HDL available that is a guess. 


16 bits is I suppose possible in a uart, but I have never seen one, usually the things are around 8 bits (Sometimes 9, occasionally 5/6/7).

No idea about that NuSMV thing, but it produces ugly looking state diagrams.