Automatic market coversion is for paying for goods or services on websites which accept WebMoney.
For example: You want to deposit your Skype account with WebMoney. You want to pay 5 WMZ ($5), but you have no money in your WMZ purse. If that, WebMoney will ask you to choose which purse you want to convert from. You can choose another type of purse and pay from it.
But I think automatic conversion for WMX purses is not available now. It will be available in the near future, when their exchange will have sufficient volume of orders.