Dummy
All the previous posts are correct...
Basically what you want is to have all the components that need good matching under the exactly same conditions, that's why, in order to avoid possible process etching effects in the border components, you place dummies in order to have them affected, and avoid having active components being affected...
That way, no active component is affected by the process and all of them see exactly the same neighbor conditions around them...
Hope this helps to clarify