I think I intended the third option to include all comma-separated lists, including:

a, b, c = x  # No brackets
(a, b, c) = x  # Round brackets
[a, b, c] = x  # Square brackets
a, = x  # Single target with comma

Perhaps something like this would be clearer:

* If the target list is
    * a comma-separated list of targets, with or without brackets, or
    * a single target with a trailing comma, with or without brackets, or
    * a single target in square brackets,
  the object must be . . .

The empty target list case could also be merged. Then the only differentiation is between a single target (no comma, optional round brackets) and iterable unpacking.