Neumann Boundary Condition OGS application

I am using OGS 6.1 to construct a GW flow model. I used a Dirichlet boundary condition at the center of the top of the model domain, and Neumann boundary condition at the sides. Rest of the surfaces are set to No Flow boundary condition.
Htop = 0.2m
QLeft = QRight = -1 x 10^-4

But I am facing some doubts regarding the application of Neumann Boundary. Please clear the following issues:
i. What does the sign (+ or -) of this Neumann Value in OGS, indicate?
ii. What physical quantity does this Neumann value in OGS signify- Flux or Gradient?
iii. How the calculation of Head Values after simulation occurs with this Neumann Boundary? Whether it uses gradient to calculate the head from the Center of the model top, or it uses the gradient value for each and every single node present on the boundary?

If available, please suggest some document related to the application of Neumann Boundary Condition in OGS.