see at item level you need to set the credit hrs and contact hrs
for example if the coures is going to be conducted only for 2 days then at item level you can put 16 credits and 16 contact hrs. So that it will be set by default. Once it is done you shouldn't change it.
so that you can schedule the classes
but after scheduling the classes you shouldn't go back and alter the changes at item level. It will affect the credits system.
so initially you set what you want.