What is meant by "tag SNP"?

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.