] gives the maximum number of indices needed to specify any part of expr, plus one.
Raw objects have depth 1.
The computation of Depth does not include heads of expressions.
See the Mathematica book: Section 2.1.7.
See also: TensorRank, Level, LeafCount, Length, Nest.
You can think of expressions as having a depth which is equal to the maximum number of levels shown by TreeForm. In general, -n in an expression is defined to consist of all subexpressions whose depth is n.
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