What is meant by "tag SNP"?Updated a month ago

Scientists have discovered over 100 million SNPs that occur across the world's population. There is no genetic testing technology that can detect that many potential variants in a single test array. Therefore the genome is sampled initially by genetic wide association studies (GWAS) to find likely hotspots where a more focused search can then be performed. Even then, scientists do not know exactly where to look for the actual functional mutation(s) that may be causing a change in enzyme activity as most SNPs are innocent and do not cause any change to the enzyme. Tag SNPs evolved as a convenient and economical way to bookmark or tag a SNP that gets inherited together with the (yet undiscovered) functional mutation or group of mutations. A tag SNP is similar to an innocent bystander. They are not the guilty party, but almost always are found nearby.