Lets say you drill a hole in a panel to run a wire and want to protect it. Digi-Key has many different products for protecting cables.
Two options that will be discussed here are Hole Grommets and Edging Grommets.
A Hole Grommet is the part to use for individual cables or wires. 
The main specs to consider are:
Panel Thickness 
Panel Cutout- Hole size: 
Inside diameter of the Grommet: 
Edging Grommets are a little Different. 
They are made to cover the edge of a panel. They also could be used as a hole Grommets if the cutout is square or bigger that what a Hole grommet can fill.
These are chosen by:
Slotted Edging
Solid Edging
Panel Thickness
Length
This is chosen by Panel Cutout Dimensions