For Combined Units consisting entirely of Attachable Models, Matching or Mismatching Bases are determined differently since there are no Rank-and-File models. In these cases, the owner of the unit decides when the unit is first formed, which model's base is used in place of Rank-and-File models. Models with the same base, or bases that are a multiple of it, are considered Matching.
If the chosen model leaves the unit or is removed as a casualty, a new model must be chosen. If possible, this choice must be made so that as few models as possible change their status regarding Matching or Mismatching, normally this means choosing a model with the same base as the model that left the unit.