A proprietary identifier used by the IPD (International Performers Database). It seems they are not public but it might still be worthwhile allowing to add them if known.

• https://www.wipo.int/export/sites/www/meetings/en/2011/wipo_cr_doc_ge_11/pdf/collective.pdf#page=9
• https://www.ppluk.com/membership/more-information/ipn/
• https://metamusic.ca/procedure/identify-creative-contributors-accurately#get-or-retrieve-your-ipn

PPL partnership with Sound Credit

• https://www.prsformusic.com/m-magazine/news/ppl-partners-with-sound-credit-for-improved-music-credits/
• https://blog.soundcredit.com/post/Sound-Credit-and-PPL-partnership