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