Search Regular Credits while contributing?

When adding Cast or Self credits to a Series, I see a new-ish search box at the top of the "Regulars" box.  Does anyone know how to use this, or what it is supposed to do?  I haven't been able to do anything with it.  Tried desktop and tablet, and both Firefox and Chrome.  You can type text into it, but nothing ever happens.

PS: the period after the word 'regular' in both cases seems very odd.