What makes an iteration candidate for inclusion in COMMON_SNAPSHOT.ITERATIONS ?

Some of our users have noted that not all the expected iterations appear in BIRT report parameter list for one of their custom reports.   My colleague (adrianof) found that not all iterations existing in the CCM database get included into the  COMMON_SNAPSHOT.ITERATIONS.   DW Jobs show no issues.

I looked at other the CCM databases and found this anomaly there as well.

What is the criteria used when adding/updating this table ?   I.e. the specific query or REST call.

Thanks.