Second Artifact in SAML Bindings doc

Hi,

in section 3.6.8 of"Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0", "Example SAML Message Exchange Using HTTP Artifact" is described.

I have a question.

At step 6, 7 and 8, a second Artifact is introduced in case of unspecified interactions. I think those interactions of step 6-8 are
"optional" or can be changed/added with any other interactions. Is my understanding correct? I believe they do not mean "MUST".

Thanks in advance for any clarification.

-Takashi