The Contributor Agreement is the agreement of the parts author to license official parts authored by them to the Creative Commons Attribution License (CCAL).
The CCAL was chosen as the license of the official library due to it allowing users to redistribute parts and, by extension, including unofficial parts within model files, along with other benefits. However, the relicensing process was long and took much effort from the Steering Committee, which tracked down the old parts authors and asked them for relicensing. Most authors were able to be contacted and most, if not all, of them agreed to the new terms. However, the committee was unable to contact several authors, leaving their parts out of the Contributor Agreement.
Since then, parts authors who had affirmed the agreement remodeled the parts and the old versions were replaced by the ones licensed under the CCAL, especially by Guy Vivan, William Howard and Philippe Hurbain. As of late 2009, every Non-CA part file within the official library was remodeled. However, several remain on the Parts Tracker as they have not been certified yet.