per @epriestley:

Carly is somewhere in Phabricator with an enormous URL (maybe Diffusion, or future pages in Facts, which are more likely to have a lot of query parameters). She presses a key like "u" and the page pops up a dialog with a short version of the current page URL she can share.
(I'm not sure how useful this really is, but it was one of the original motivating use cases; it is likely to get more useful after Facts is built.)