The rsID number is a unique label ("rs" followed by a number) used by researchers and databases to identify a specific SNP (Single Nucleotide Polymorphism). It stands for Reference SNP cluster ID and is the naming convention used for most SNPs.
When researchers identify a SNP, they send a report (which includes the sequence immediately surrounding the SNP) to the dbSNP database. If overlapping reports are sent in, they are merged into the same, non-redundant Reference SNP cluster, which is assigned a unique rsID.
If a probe on our genotyping platform doesn't correspond to a SNP with a clear rsID, or the probe is assaying a DNA change that is not a known SNP (i.e. it doesn't have an rsID), then that marker is usually assigned an "internal" id ("i" followed by a number). Our researchers may have included some of these "custom" SNPs on our genotyping platform in order to maximize the number of 23andMe features available to customers, as well as to offer flexibility for future research.
In general, many SNPs labeled with an "internal" id in the Raw Data feature may not have a corresponding rsID in outside scientific literature or other third party services.
Genome-wide association studies linking SNPs to traits or conditions usually report their results by rsID. The rsID numbers for SNPs in Health and Traits reports can be found in the Scientific Details section.