V1 is called the decision speed, at which the decision to abort or to continue a takeoff must have been made.
Vmcg is the lower limit for V1 and the lower of
a) VR or
b) VMBE or
c) the speed at which the accelerate stop distance equals runway lengh available (+stopway)
the upper limit for V1.
However, V1, is critical for the length of the takeoff distance and the length of the accelerated stop distance.
A lower V1 results in a longer takeoff distance (longer acceleration with engine out) but shorter accelerate stop distance (braking starts earlier).
A higher V1 has opposite effects, takeoff distance becomes shorter and accelerate stop distance becomes longer.
In case the choosen V1 is such that takeoff distance equals the accelerate stop distance, it is called a balanced V1 or balanced field length.
Balanced field usually results in the highest takeoff weight achievable, unless a clearway or stopway is available to further increase the takeoff weight.
So generally, for an actual flight the V1 is choosen from an available range of V1 between minimum and maximum V1. During takeoff, there is only one V1 used, the choosen or calculated V1 according to the requirements for the given takeoff.
On a wet runway a lower V1 is generally more desirable as poor braking conditions may exist and increasing the available stopping distance will increase the safety margin.