What is defined by the MeshCoff option?
Siesta uses a finite 3D grid for the calculation of some integrals and the representation of charge densities and potentials. Its fineness is determined by its plane-wave cutoff, as given by the MeshCutoff option. It means that all periodic plane waves with kinetic energy lower than this cutoff can be represented in the grid without aliasing. In turn, this implies that if a function (e.g. the density or the effective potential) is an expansion of only these plane waves, it can be Fourier transformed back and forth without any approximation.
The MeshCutoff used by SIESTA is given but the actual numbers of division along lattice vectors accepted by the system. This parameter should be converged before starting production calculations.
Siesta prints out explicitly the MeshCutoff used. Search in the output for e.g.:
InitMesh: MESH = 40 x 40 x 40 = 64000
InitMesh: Mesh cutoff (required, used) = 350.000 425.493 Ry