Lets understand the definition of co-payment first that will help you understand it better :
The percentage or amount of eligible expenses that the insured pays after the deductible.
Please visit :
www.path2usa.com/Insuranc...urance.htm and look for the terminology defined the on one the left side related link, that explains it with an example as well.
Inbound plan is such a plan where you need not to pay any Co-pay unlike other plan where you pay co-pay as well other then the deductible.
Again, each plans has their own significance and features so one need to look for all commonly applicable features while deciding.