It's mentioned in the "Cross Reference Examples" section going back to at least v.7.
"You can use the </> building block as a substitute for <Default Para Font> in cross-reference definitions."
You can use the </> building block practically anywhere that they are used to turn off (reset) previously applied character tag building blocks: variables, indices, markers, x-refs. (Except in auto-numbers). It's a lot easier and more succinct to enter and was often needed with the 255 character limit in some items.