A Constant Volume Chart a.k.a Equivalent Volume Chart is a chart, it can be a bar or candle chart, where each bar is made up of price changes that occurred when a constant (fixed) number of contracts were traded.
On a Volume Chart, each time the specified number of contracts are traded, a new bar or candle is created from which the Open, High, Low, and Close prices are collected for that bar. A special case is when volume is set to 1. This creates tick chart on which trading is possible.
MT4 does not store tick data therefore historical bars are an approximation. It is quite good when source bar volume is less than resulting equivalent bar volume (therefore the best is making it from 1 min chart).
This video shows a demonstration of usage of the off-line charts.
Some frequently asked questions about application of off-line charts are answered here Off-line charts FAQ.pdf.