Variables allow you to enter a string that can be inserted in topics, topic titles, master pages, snippets, index entries and a table of contents. They are used where the string could change in the future. Company and product names are good examples. Instead of having of having to edit every place the company or product name is used, you insert a variable. When the value of that variable changes, you change it once and it cascades to wherever you have used it.
To add a variable, click the + icon and enter the name and default value for the variable. To edit or delete it, click the menu that appears when you hover over the variable. You can also double click to edit.
The name of the variable no longer has to include an underscore.
To use the variable, just drag it to where you want in the topic or press CTRL + 1 to display a dialog with the variables listed.
Having created your variables with their default values, the next stage is to create the values that will vary.
The Sets are selected when you configure an output and that is also where the enter the different values. Select Output > Output Settings from the menu and you will see Variable Sets in the left toolbar.
Let's say the default values are for US users, in Output Settings you click Variable Sets and the + icon and create UK Values. All the variables you have created will be listed and you can apply the UK values.
Repeat the process for all the sets you require.
Later if you tick the check boxes, a Reset button allows you to revert all the values to the default values so that you can start again.