DITA doesn't have an actual variables element, but you can use keyref to accomplish the same thing.

A variable is a symbol that represents something else. You might use the variable product_name so that if you're product name switches, you don't have to update all instances of the product name in your documentation. You can just update what the variable product_name points to.

In your map file, add the following:
  <keydef keys="product_name">
   <topicmeta>
    <keywords>
     <keyword>ACME</keyword>
    </keywords>
   </topicmeta>
  </keydef>
In your topic file, reference the variable with the following:
Congratulations on the purchase of <keyword keyref="product_name"/>. 
You can event put attributes on the keyword so that different product names appear based on different conditional processing.

 <keydef keys="product_name">
   <topicmeta>
    <keywords>
     <keyword audience="UK">ACME</keyword>
     <keyword audience="USA">Gizmo</keyword>
    </keywords>
   </topicmeta>
  </keydef>

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