This introduces a bit of uncertainty into the process. Applications can ignore novel tags. This was the approach typically taken by browser vendors during the height of the browser wars. Leading browser vendors merrily defined new tags with each release of their product, and each browser distribution quietly ignored those tags it didn't understand. This approach is safe but not very satisfactory.
Organizations can standardize on a set of tags.
This approach cleverly sidesteps the entire problem. This is actually the right solution for many problems. Sales orders, for example, fit a well-defined pattern. Applications can try to figure out what to do with novel tags. There are several ways to solve this problem.