To discuss the modeling of unresolved scales, first the unresolved scales must be classified. They fall into two groups: resolved sub-filter scales (SFS), and sub-grid scales(SGS).
The resolved sub-filter scales represent the scales with wave numbers larger than the cutoff wave number , but whose effects are dampened by the filter.  Resolved sub-filter scales only exist when filters non-local in wave-space are used (such as a box or Gaussian filter).  These resolved sub-filter scales must be modeled using filter reconstruction.
Sub-grid scales are any scales that are smaller than the cutoff filter width .  The form of the SGS model depends on the filter implementation.  As mentioned in the Numerical methods for LES section, if implicit LES is considered,  no SGS model is implemented and the numerical effects of the discretization are assumed to mimic the physics of the unresolved turbulent motions.