From BlenderWiki

Select Menu

The Select menu in Edit mode for meshes is quite large – here we will discuss about the “general” options, the menu entries concerning specifically edges or faces are described in their dedicated pages.

Note that these select tools are regrouped in this menu, but that you will find some in different other popup menus (e.g. Vertex Path and Inverse – renamed Select Swap! – are both in the Specials menu (W), edges select options are in the Edge Specials menu (CtrlE), etc.). Quite confusing, so just remember that all are regrouped in the Select menu of the 3D View windows headers…

Region to Loop

Loop to Region

Edge Ring

Edge Loop

Sharp Edges

Non-Manifold

These options concern edges selection.

Non-Triangles/Quads

Quads

Triangles

Linked Flat Faces

These options concern faces selection.

Random...

Inverse

Select/Deselect All

Border Select

All these options have the same meaning and behavior as in Object mode (and the specificities of Border Select in Edit mode have already been discussed here).

Vertex Path

This tool will select a “path” of vertices between the two already selected. This implies that you must be in Vertex select mode, and have exactly two selected vertices before using it.

You have two options as path-finding algorithm:

• Edge Length selects the shortest path between the two vertices.
• Topological selects the path based on the topology of the mesh. Note that I don’t know how exactly it works – the results seem quite strange sometimes, and modifying the vertices’ position (without touching to the topology, i.e. without modifying edges or faces defined by these vertices, nor adding some) modify the result of this option…

Note that this option is also available from the Vertex Specials menu (CtrlV).

Linked Vertices

This is quite a simple and useful tool, especially when working with objects made of several distinct parts: it selects all vertices that are “linked” (edge-connected) to those already selected. This allow you to cleanly and simply select one whole part of your mesh, just starting with one vertex.

Less and More

These two options are complementary. Their purpose, based on current selected elements, is to reduce or enlarge this selection.

The algorithm seems to be:

• Less: for each selected vertex (either explicitly or implicitly, in Edge or Face select mode), if all vertices linked to this vertex are selected, keep it selected. For all other selected vertices, de-select them.
• More: for each selected vertex, select all its linked vertices.

This implies two points:

• First, when all vertices of a mesh (or mesh part) are selected, nothing will happen (as for Less, all linked vertices are always selected, and of course, More can’t add any). Conversely, the same goes when no vertex is selected.
• Second, these tools will never “go outside” of a group of linked vertices (i.e. they affect whole parts of a mesh, exactly as the Linked Vertices option).

Similar to Selection...

This is another quite complex option. It will pop-up a different sub-menu, depending on which select mode you are using:

In Vertex select mode

You have three options:

• Normal: Selects all vertices that have (nearly) the same normal as those currently selected (note: I don’t know how exactly this work – perhaps by checking the dot product?).
• Face Users: Selects all vertices that belong to exactly the same number of faces as those already selected. For example, assuming you have only one selected vertice, if this one does not belong to any face, this option will select all vertices not involved in faces. If it belongs to exactly four faces (e.g. is at the center of a subdivided quad), all vertices at in the same situation will be selected, etc…
• Shared Vertex Groups: Selects all vertices sharing one or more vertex groups with those already selected.

In Edge select mode

Here you have seven options:

• Length: Selects all edges that have (nearly) the same length as those already selected. The tolerance seems to be ±0.5%.
• Direction: Selects all edges that have (nearly) the same direction (angle) as those already selected. The tolerance seems to be ±0.5°.
• Face Users: Selects all edges that belong to exactly the same number of faces as those already selected. This is the same as for Vertex select mode described above…
• Face Angle: Selects all edges that are between two faces forming (nearly) the same angle, as with those already selected. Here again, the tolerance seems to be ±0.5°.
• Crease: Selects all edges that have (nearly) the same Crease value as those already selected. The Crease value is a setting used by the Subsurf modifier. The tolerance seems to be ±0.01.
• Seam: Selects all edges that have the same Seam state as those already selected. Seam is a two-states setting used in UV-texturing.
• Sharp: Selects all edges that have the same Sharp state as those already selected. Sharp is a two-state setting (a flag) used by the EdgeSplit modifier.

In Face select mode

And six options for faces:

• Material: Selects all faces that use the same material as those already selected.
• Image: Selects all faces that use the same UV-texture as those already selected (see UV-texturing pages).
• Area: Selects all faces that have (nearly) the same area as those already selected. Here again, the tolerance seems to be ±0.5%.
• Perimeter: Selects all faces that have (nearly) the same perimeter as thos already selected. As you might have guessed, the tolerance seems to be ±0.5%.
• Normal: Selects all faces that have (nearly) the same normal as those selected (note: I suppose the same algorithm is used here as with the vertices’ normals…). This is a way to select faces that have the same orientation (angle).
• Co-Planar: Selects all faces that are (nearly) in the same plane as those selected. I think the tolerances are those of the Normal option for orientation (angle), and ±0.01BU for the gap between planes…