The n+ and n- are the positive and negative nodes, respectively. Current flow is from the positive node, through the source, to the negative node. The parameters nc+ and nc- are the positive and negative controlling nodes, respectively.
In the first form, if the expr is a constant, it represents the transconductance in siemens. If no expression is given, a unit constant value is assumed. Otherwise, the expr computes the source current, where the variable ``x'' if used in the expr is taken to be the controlling voltage (v(nc+,nc-)). In this case only, the pwl construct if used in the expr takes as its input variable the value of ``x'' rather than time, thus a piecewise linear transfer function can be implemented using a pwl statement. The second form is similar, but ``x'' is not defined. The keywords ``function'' and ``cur'' are equivalent. The third form allows use of the SPICE2 poly construct.
More information on the function specification can be found in 2.15, and the poly specification is described in 2.15.2.