Consider the spin echo sequence to which a 90 pulse has been added at the end: 90(x)

Question:

Consider the spin echo sequence to which a 90° pulse has been added at the end:
90°(x) - τ - 180°(x) - τ - 90°(ϕ).
The axis about which the pulse is applied is given in brackets after the flip angle. Explain in what way the outcome is different depending on whether the phase ϕ of the pulse is chosen to be x, y, -x or -y.