MODELLING CONCEPTS
Linear Connection Reduction.
Occasionally there will be a need to transition from a very dense area of a mesh to an area which does not require as much weight of geometry. A very flat area is required as a good candidate for linear connection reduction.
As the topology in the transition area varies from the surrounding geometry the transitions need to happen within local geometries rather than in border areas as heavy distortions would occur on corners and edges.
The transitions are effectively partial insets which don't create entire border loops so the geometry they create is still considered to be part of the local geometry.
Any of the techniques described in the diagram below are usually reserved for connecting mismatching meshes together. Hands, fingers and feet on character models will often require some polygon reduction to ensure that limbs and torsos do not have excessive geometry.
Linear Connection Reduction
It is very rare to use any of these techniques while modelling almost anything else as the topology they produce will create pinching on anything other than a perfectly flat surface. It is almost always preferable to tolerate a higher density of geometry than it is to use Linear connection reduction. The "curve" tool can be used to correctly distribute edges rather than remove them.
On the rare occasions they are used it is normally 3 to 1. The others are problematic.
When 5-3 is used, the version on the left is preferred for subdivision Framework Meshes and the alternative version on the right is preferred when creating game models as it is lighter.
Linear connection reduction can be useful to redirect the flow within a topology but again, there are usually better methods to do this.
If you do want to use these techniques you will almost always need a perfectly flat area of your model. Most objects in the real world are never perfectly flat anywhere so these techniques are often surplus to requirements.