Com Unit Is are similar to modern Cell Phones. Simpler models use a touchpad to dial your calls; higher rated models take voice commands. Some have special attachments to give them limited Multicorder activity or to send text messages to people.
It is possible, but slow and often malfunctionable, to check your C-Mail by Com Unit I.
Com Unit II have greatly enhanced private communications range. While a Com Unit I can, in theory, call anywhere in the complex, the cell system often goes down, whereas, Com Unit IIs can call any functioning Com I or II within their range, barring disaster. This also allows private, harder to spy on conversations. (Whereas, in theory, Friend Computer listens to all Com I calls. But probably isn't paying attention.)
Com Unit III are typically not very easily transported, but are as powerful as an FM broadcast tower, and are often put into vehicles for work in the field, especially in OUT sector.