] prints with boxes inside expr shown in explicit two-dimensional or other form.
In ordinary StandardForm output, boxes such as SubscriptBox are shown literally. In DisplayForm they are shown as explicit two-dimensional constructs.
Example: DisplayForm[SubscriptBox["x", "y"]].
DisplayForm acts as a "wrapper", which affects printing, but not evaluation.
See the Mathematica book: Section 2.8.10.
See also: FullForm, ToExpression, ToBoxes.
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