This is the complex shear modulus divided by the sine of the phase angle when tested at 10 rad/s loading frequency at high pavement temperatures. It is used to calculate performance-related criteria in accordance with various bitumen specifications such as; AASHTO M-320 and M-332 or equivalents by ASTM, EN, DIN, etc. Dividing by the sine of the phase angle provided a preference to bitumen with elastic behavior. With no elasticity the sine of delta at 90 degrees is 1 which in unfavourable to pavement performance.  With a phase of 45 degrees, it’s 0.707 so when dividing by a smaller number bumps up the PG result so you can use a lower modulus binder.