Custom fields can be used to display additional information in documents that are necessary and specific to your use case.

Custom fields can be managed by going to My Company -> Settings -> Sales/Purchases -> Custom Fields

Possible applications of the custom fields are displaying in documents:
- reference to other documents used in your workflow
- project/work locations
- representative's contact information
- tracking numbers
- additional information in other languages etc.

Customs fields can also be used for keeping internal information such as internal reference numbers etc.. For this purpose tick the box Internal Use in a custom field settings and then such information will only be visible in the program, but will not be reflected on printed versions of documents and PDF files.

Mark a custom field as default and it definitely shows up on all documents.

A custom field could be active or inactive. If the field is active, it is available in documents and usable. In case you temporarily do not want to use any of the customs fields, mark it as inactive.