In a nutshell the executive summary is an elaborate version of the 510(k) summary, which is available to the public. For example, the predicate device is identified in the 510(k) summary but the executive summary includes a comparison table between the new and the predicate device.
Another example, the 510(k) summary includes non-clinical performance data but the executive summary includes more details (standards, spec, etc.).
I view the executive summary as in introduction to the reviewer while the 510(k) summary is the abbreviated version for the public. I am not aware of any type of mandatory information in the executive summary which is not mentioned in the 510(k) summary.