The definition of potential difference in the coloured box mentions unit positive charge only.Why is it so?What is unit positive charge

If two charged bodies are in contact, the charge starts flowing from one conductor to other. The electric condition, that determines the flow of charge from one conductor to other in contact, is the electric potential.  Earth is a conductor that can hold an infinite charge and can give infinite charge without changing its potential.

Its potential is taken as zero potential. Thus, the positively charged conductor has more positive potential than the earth. A  negatively charged conductor has negative potential less than that of the earth.The electric potential difference between two points in an electric circuit is the work done in moving a unit positive charge from one point to the other.

To simply the definition, unit positive charge is written. But this definition applies to both positive and negative charges. To avoid the confusion of the signs, unit positive charge is taken. 

Also, work done on moving the positive charge depends on the direction of electric field in which it is moved. If it is moved against the direction of the field, force is exerted on it thus work has to be done against the repulsive electrostatic force and thus the work done is negative. Similarly if it is moved in the direction of the field then the work done is positive.

The unit positive charge means a charge amount of +1 coulomb charge.