LTSpice arbitrary voltage block (bv) simulating full wave bridge rectifier
You can take the voltage of a node, a voltage difference between nodes or a current through a device as input of a B-source.

The syntax to access these are:


V(node) for a voltage of a node, so V(line) in your example
V(node1, node2) for a voltage difference between node 1 and node 2 (node 1 - node 2)
I(component) for example I(V1) - note however, that there is no immediate feedback as detailed in the help


Detailed description for device currents:


  However, it is assumed that the circuit element current is varying
  quasi-statically, that is, there is no instantaneous feedback between
  the current through the referenced device and the behavioral source
  output. Similarly, any ac component of such a device current is
  assumed to be zero in a small signal linear .AC analysis.


The I() and V() syntax is required as you can create a node which is named R1 and a resistor which is named R1 (I do not promote such a naming scheme, but it is possible), so LTSpice cannot easily decide which one you want.