Hello,
Let's details elements, step by step :
1. Product Custom Field is for e-commerce manager, this means that :
- Only manager can define it from backend
- Will be only display in frontend to inform customer
=> If the product custom field isn't displayed for the manager (in backend product setting), it's because it don't have to, I mean there is a restriction, like Nicolas said certainly an association of the product custom field to category, as result if the product isn't in this category, the custom field won't be displayed at all.
2. Item Custom field is for get information from customer :
- Will be displayed to be filled in frontend (and add where you want it from custom field settings)
=> Display part options (general, cart & mail)
Hope this will be more clear in your mind.
Regards