Integral of pressure constraint in dense suspension tutorial

Hi,

I'm trying to figure out the meaning of the weak constraint on the integral of the pressure in the tutorial Two-phase modeling of a dense suspension.

Mainly, a total force is defined via Fp_tot = intop1(p) where p is the pressure and then a weak constraint (at a point) is added via test(Fp_tot); this constraint ensuring that the integral of the pressure is equal to zero.

Does anyone know why this procedure is adopted? I thought it might be because no pressure is defined as a boundary condition, but I'm unsure about this particular procedure with the addition of a weak constraint.

Thanks in advance,

Best, J