DimensionConstraint

Whether a PageItem's dimension (or the margin on either side of it) stays fixed relative to its parent, or is free to vary, under InDesign's object-based layout rules ("Liquid Layout").

Corresponds exactly to InDesign's own DimensionsConstraints enum (ExtendScript), which has only these two values - confirmed against the InDesign scripting API reference, since this isn't otherwise documented anywhere in the IDML files themselves.