The Number format... property allows you to specify the format of numbers. When you click on the option, this is the menu that opens:

You can use the menu to choose between the four main formats, Number, Currency, Scientific, Percentage, or you can add a Custom format. When you select Number (Custom) or Currency (Custom), you have a few options to create the format of your choice.

Once you specify a default format, Tableau will use it every time you use the Measure. The last two default properties are about aggregation.