MODELLING CONCEPTS
Edge.
Two vertices are connected by an edge.
Understanding the different arrangements and flows of edges is one of the most important aspects of subdivision surface modelling. Multiple edges, connected end to end, are called a loop.
They can either connect all the way around a section of geometry, which we call a full loop or they may terminate for a variety of reasons and we call this a partial loop. We normally endeavour to make sure that most of the loops in a mesh are full loops. The existence of partial loops inside a local geometry can indicate faulty topology somewhere on your mesh; often due to non-manifold constructs, triangles or n-gons.
Partial loops certainly do exist in may forms in a subdivision Surface mesh and usually indicate a change of direction of flow.
Maintaining good loop structure is most certainly not always achievable without compromise which means that subdivision ideals are failing in another area. Subdivision surface modelling is about finding the best balance between the ideals (2-manifold, all quad, mostly full loops) and the realities of other abstracts such as weight of geometry, reduction techniques and unusual shaping.
Edges selected on a mesh