In this sof ticket, Where do I find PingFederate's "Relying Party"? (To be set in "AppliesTo" of the SAML RST), we learn where to set an SP's "PARTNER SERVICE IDENTIFIER" which corresponds to the "AppliesTo" element in the STS soap request for saml token.
I am using PingFederate to do SSO to my SharePoint online site: https://mysite.sharepoint.com
What should the PARTNER SERVICE IDENTIFIER be set to when using SharePoint online?
Finally found the answer from a PingFederate PDF document that was pretty hard to find.
https://docs.pingidentity.com/bundle/ix_m_downloadDocumentation/page/PF610O365IntegrationNote.pdf
The value to use is: urn:federation:MicrosoftOnline