So, today's puzzle isn't really a code challenge, but more of a trivia contest. The first person to both answer my question, and explain the answer, wins, um, respect and Kudo(tm) points, but nothing else. Ready?

By default, CFML tags will either look like this, <cffoo> or like this, <cf_foo>. However, there is one more format that works as well. Tell me what the format is, how it works, and why it exists.