Only consider nested expressions with head List:

By default, nested expressions with any head are considered arrays:

Only consider nested expressions with head List:

By default, rectangular nested expressions with any head are considered arrays:

Sum inside the outermost head:

Sum inside both f and g:

By default, Total only sums inside list representations and associations:

Sum inside several specific heads:

Sum inside any head:

Allow any head at each level of the array:

An array has a consistent head at each level:

Completely ignore heads, looking only at the tree structure of the expression:

Treat the heads g and h as the same head:

Without this stronger equivalence, the expression is considered one-dimensional:

Create a version of MatrixQ that only accepts list matrices:

Create several expressions to test:

The function matrixQ gives True only for listMat, which both is a matrix and has head List:

MatrixQ gives True for matrices of any known array type:

Once a head is excluded, no nested expressions are tested against the value of AllowedHeads:

If both f and List are allowed, then the inner list is tested and summed:

The meaning of AllowedHeads->Automatic may vary from function to function: