they review eligibility, but not necessarily the relationship. they leave the relationship to the overseas visa offices since their officers will have a better understanding of cultural norms in their service region.
applicant eligibility involves many other things besides the bona fides of the relationship, and it's those things that CPC-M quickly reviews before transferring the file. if during the review they find any potential issues, he/she will mark eligibility as "review required" before the file transfer.
don't worry, this is a good sign. you're either going to get PPR soon, or a remedical (if you haven't done one already) followed by PPR.