CLI overstamping, number overstamping or CallerID overstamping are the same terminology.
Number overstamping allows you to present your PSTN phone number (aka, your house phone number) to your callee when making out-going calls. This way your real number is shown to the callee and is more likely to be answered. When you call friends and families using ClicknCall they can easily identify your phone number and take your call.
CLI overstamping is also very useful when making business call as it presents a real PSTN number and less likely to be "screened".
You can set up multiple numbers such as your home phone, your work phone or mobile phone as the overstamping number(or CallerID). Only one number can be used as the default CallerID at any one time. You can easily set which number to use as the default CallerID at a click of a button.