Crystallographers fit the model in the electron density map, while NMR spectroscopists try to come up with a model that explains the observed distances as good as possible. Crystallographic models consist for each atom of Cartesian x , y, z coordinates and a so called B-factor that indicates the mobility of the atom. NMR models have also Cartesian coordinates but describe mobility and lack of sufficient data in terms of alternative models. Obviously, the larger the number of observations the better the quality of the model is. Each crystallographic observation has contributions from all atoms, while NOE's only bear upon the local protein environment.
Crystallographers always describe the global quality of the model by the R-factor or residual index.
The lower the R-factor the better the model explains the observations. At least the same number of experimental observations are required for the system to be mathematically determined, a condition often experimentally not met. Luckily, our knowledge about structural aspects that are invariant with respect to protein conformation can be added into the equation in terms of restraints, making the problem mathematically tractable. Such knowledge consists of bond lengths and angles, the shortest possible distance between to non-covalently bound atoms, etc.