both A and B give the area under the integral curve for y(x) where y is zero at x = 0 and dy/dx is always positive.

dA = y dx

dB =

We can call that area also U but

dU = where stands for inexact differential. Inexact differentials are used in thermodynamics where only the sum of the two differentials have meaning, not their individual values. The individual values are path dependant. B stands for mechanical and A for thermal energy.