The stages of an application are:
R10 - completness check
Criminality - Based on your PCCs, and if your are inland, will be pending until RCMP is received
Medicals - Is done in the first 39-45 days, usually around R10
Eligibility - Only after Medicals and R10 will the eligibility start.
Security - Only after eligibility is conclusively passed by an officer, will the security commence.
When an agent says that the eligibility is under review, it merely means it is in progress.
The eligibility takes place in 2 stages. First an assistant / analyst will review the documents and make a recommendation. This can be Recommended passed, Review Required, Met recommended Met etc. To know if you have a RR, you will need to see the GCMS notes.
Once the recommendation is done, the officer will review and make a final determination. This is when your eligibility is passed.
Under review can mean either it is in progress, or the first stage is done.
Also a RR by itself is not a reason to panic. I wrote an exhaustive post and posting it here.
Hope this helps!!