Approval Status is a list not indicated when navigating to Customization > Lists, Records, & Fields > Lists. This article shows the Internal IDs associated to the statuses of a custom transaction body field with type of List/Record and using 'Approval Status' for the List/Record of the field. The value of the custom transaction body field can be set using nlapiSetFieldValue and retrieved using nlapiGetFieldValue via scripting.
Here are the Internal IDs and the corresponding text values that are seen in the UI:
1 - Pending Approval
2 - Approved
3 - Rejected