The master version is the first and most powerful version of your mailing in Taxi. The master version controls inheritance, which means it is connected to all other versions and that changes in the master version will take effect in all other versions. So if you have a lot of versions of your mailing and you need to make a change, you only have to do it once. For Example: You have 10 versions of a newsletter for 10 customer groups. Each version has a 15% discount on a product relevant for that group. At the last minute you need to increase the discount from 15% to 20% and add in a new phone number. Instead of changing the % and the phone number in all 10 versions, you just need to change it in the master version and all the others will update automatically.