In some cases, an IdP needs to know who the RP is in order to actually allow a session. Below are two screenshots of a SAML use case where a user is trying to log into the Feide IdP from two different RPs.

RP does not have a relationship with the IdP:

RP does have a relationship with the IdP:

To replicate where the RP does not have a relationship with the IdP:

1. Go to https://edusign.sunet.se/ and click on "Access through your institution"
2. Type in "Oslo" and select "Feide"

To replicate where the RP does have a relationship with the IdP:

1. Go to https://iopscience.iop.org/article/10.35848/1882-0786/abdcd7 and click on "Access through your institution"
2. Type in "Oslo" and select "Feide"