Question

relative velocity object a with respect to object b

Answer 1

Relative velocity is a vector. It may be subtract or addition depending upon the direction.

Answer 2

If two objects are moving in parallel their relative velocity can be calculated. For example, two cars are moving in the same direction along a road. Car A is travelling at +30 m/s and Car B is travelling at +20 m/s. Their relative velocity is 30 – 20 = +10 m/s.