We probably not referring the same versions of serviceNow.
More like your instance has some customization that includes that tab.
But back to the original problem, you should get the Variables option after you click on the Show related fields (very last) entry - assuming the dependent field points to a table extending Task (which does seem to be the case).
When configuring a task, at one point (in my version), you can set the condition at which that task will be created !!!!! My condition will be based on the value of a given variable.
