Let's try to make it simplifier to read here:
At stage 1, if everything goes well (sponsor meets eligibility and relationship is believed to be genuine) :
Sponsorship Eligibility Letter --> Sponsor
Application for PR Eligibility Letter --> Applicant / the one being sponsored (the real AIP)
[if relationship is in doubt, but sponsorship meets eligibility as a PR/Citizen with valid status and is not under another spousal sponsorship within 5 years, the sponsor can still receive the sponsorship eligibility letter but the applicant might not receive AIP, or letter requesting further documents, or being transferred to local office for interview.]
*In my case, these two letters / email will be sent at the same time, but just to different email address as one to sponsor, and the other to the applicant.
At Stage 2, if med/background check/police check are clear:
Decision made Letter --> Sponsor and Applicant
After DM, we still need to wait for another letter from local office to notify about landing interview, and this letter will be delivered via Canada Post instead of email.