To rename a custom field, open the data entry window, right-click the data field and choose Rename Field from the popup menu. You can also rename fields from the 'Database Setup' window (Tools=>Database Setup).
The Contents table contains the following custom fields:
Text
Custom 01, Custom 02, Custom 03
Lookup
Custom 04, Custom 05, Custom 06
Date
Custom 07, Custom 16
Numeric
Custom 08, Custom 09
Checkbox
Custom 10, Custom 11, Custom 12, Custom 13, Custom 14, Custom 15, Custom 17, Custom 18, Custom 19, Custom 20