"+" means that wind pressure direction is downwards, "-" - upwards. Overall force coefficients means resulting force that will act on your structure, whereas cp,net coefficients in zone A,B,C etc mean local pressure acting on that zone only.
In table 7.6 I understand that Φ = 0 when there is no blockage (petrol station) and Φ=1 when there is blockage (maybe a bus station ?). However, I have a few questions:
1) What does plus or minus mean ?
2) What is the Overall force coefficients?
3) What is the maximum all φ ?