Once the Applicant's file goes to the respective visa office where it will be reviewed, the applicant is given a new UCI number (please note that all applications be it a skilled worker, sponsorship, tourist visa is given a UCI number). So, after the applicant is given a UCI number that applicant can log into ecas using the UCI number and that applicant's details such as the applicant's date of birth, place of birth, last name etc. Everyone's UCI is different.
If you are the sponsor, and the file has been transferred to the visa office for review, then you are already approved as the sponsor. So, in ecas if the sponsor's UCI and details are used, it shows decision made (i.e. you are approved as an sponsor). But, if you use applicant's 's UCI and the applicant's details, then ecas will show that application has been received. Do not mix one person's UCI with another person's details.
You can also use the File number and applicant's details to log-in. Note that, when Mississauga case processing centre sent you the notice of stage 1 approval (sponsor approval), they gave a file number on the letter. This file number remains the same.