just my take but ...
Foundation basically says I've heard of ITIL, can probably name most of the parts, but I cant really say much more than 'what is an incident/problem/change etc
Practitioner means that the subject chosen for the practitioner is, or was key to my roll. Im up to a good standard of understanding in that discipline, and could quite happily talk in a greater depth about the other disciplines that feed into this.
Manager means that your either Service Support/Delivery manager, or part of a project implementing or modernising an existing ITIL based support structure.
....
As such, ITIL Foundation is just saying 'I know the basics of a best practice framework'