LedgerSMB is available under the terms of the GNU Public License version 2, in short: GPLv2 22 2 https://opensource.org/license/gpl-2-0/.
The project attaches the following meaning to this license: The copyright holders grant you the right to copy and redistribute the software. In case you make any modifications to the software you’re obligated to publish the changes if you distribute the software. You are always free to provide third party access to the API from your modified software without being required to disclose your changes.
The project considers the API to include:
Database tables
URLs with their input and output
Webservices of any kind
Function and object calls
The effect of this interpretation is that changes directly to the code base as well as inheritance of classes defined in the software constitute “making modifications”.