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 Stamp table contains the following custom fields:
Text
Custom 01, Custom 02, Custom 03, Custom 04, Custom 05, Custom 06, Custom 07, Custom 27, Custom 28
Lookup
Custom 08, Custom 09, Custom 10, Custom 11, Custom 15, Custom 16, Custom 17, Custom 29, Custom 30
Multivalue lookup
Custom 44
Memo
Custom 35
Numeric
Custom 18, Custom 19, Custom 33, Custom 34
Date
Custom 20, Custom 21, Custom 31, Custom 32
Checkbox
Custom 12, Custom 13, Custom 14, Custom 22, Custom 23, Custom 24, Custom 25, Custom 26, Custom 36, Custom 37, Custom 38, Custom 39, Custom 40, Custom 41, Custom 42, Custom 43