You have to set up a Timeline under Manage project timelines under the drop down Gear in upper right of screen.  Then you add the time line to your test schedule in the test plan.  You can set up multiple iteration schedules say for each release and then choose the one to apply to the test plan - this will pull in all the sub levels that are part of the iteration top level.  Then when you go to create TERs you will see the iteration values on the left side of the Generate TER panel.  You can multi-select values so you can create duplicate TERs for different iterations for example you might have regression TERs that you want to run through out the release.