Open/Cable Type, Blind/Blanking Type, Semi blind, Rubber Washers
Just a word to the mainly natural product 'Rubber'.
Durable, long lasting and resistant against hig temperature-changes, resistant against most solvents like petrol, diesel, oil. This is why Rubber is (was) a preferred material in the car industry.
The Open Type grommets are used in industry and car manufacturing to protect cables when feeding through a metal sheet.
The Blanking Type grommets are used to reclose drilled holes which have to be opened later again for service issues. The most common usage is to close the holes which were used for Waxoiling boxed section on the car.
Hint: If the hole is a little bit too narrow to fit the grommet, just use WD40 to get the grommet slippery. This wouldn't harm the grommet and the cables and fitting is much easier.