I am reading this introduction paper on Gromov-Witten theory, and on page 5, in second paragraph he says:
Within this locus of maps, there is a sublocus consisting of those maps which map as $2 : 1$ covers of a line in $\mathbb P^2$ . This is a $4$-dimensional locus, since we need two parameters to describe the target line, and two to describe the ramification points of the map.
I am a little confused about this. I suppose that two maps $\mathbb P^1 \to \mathbb P^2$ are equivalent if there is an automorphism $\mathbb P^1 \to \mathbb P^1$ making the diagram commutative, and he is computing the dimension of the equivalence classes. Therefore, a $2:1$ cover $\mathbb P^1 \to \mathbb P^1$ will be determined only by the branched value, hence is described by one parameter. Why he says two?