Products, questions and answers have 4 to 6 custom fields you can use to enter any kind of additional data. The purpose of these fields is not specific. Typical useful applications are:
Usage in price calculations (see onscreen-help for detailed info)
Usage in compatibility rules
Usage in custom templates for output or applying custom logic
Usage in connectors for integration with third party software
To make data entry easier, you can set your own labels of these fields in Settings -> Custom Fields.