Completely-in-canal (CIC)
Before invisible-in-canal (IIC) hearing aids, completely-in-canal (CIC) hearings were the smallest custom hearing aids available. They are designed and sculptured to fit almost entirely inside your ear canal (external auditory meatus) and are therefore almost invisible, with only the faceplate and battery drawer usually visible. Extraction cords are usually fitted to CIC hearing aids to help insert and remove them from the ear.

Benefits
Small size and low profile.
More powerful than their small size initially suggests and typically suitable for mild to severe/profound hearing losses.
The location of the microphone in the ear canal, as opposed to behind the ear, helps with:
using the telephone.
the preservation of the natural acoustics provided by the external ear (pinna) that help with localizing the direction of sound from in front and behind you.
Most manufacturers offer CIC hearing aids with both wireless and telecoil options, albeit they are slightly larger in size.

Limitations
Single omni-directional microphone that is sensitive to sounds arriving from all around you. Subsequently, they are not always best equipped for when hearing in the presence of background noise.
Ear anatomy must be of a certain shape and size to house all the electronic components inside.
Not appropriate if you have poor vision or manual dexterity.
Require more maintenance and are more susceptible to damage due to ear wax ingression inside the microphone port, which is located near the ear canal entrance.
Smaller surface area means they are more likely to:
feedback (e.g. whistle) due to acoustic leakage
work loose whilst talking and chewing, especially if you have a straight and inclined ear canal shape.
As with all custom hearing aids, CIC hearing aids may need to be ‘re-shelled’ from time to time since the ear canal cartilage can change shape and size. This is not covered under the warranty and will require new ear impressions.