Leftwards Arrow with Vertical Stroke

It marks the arrow as a partial or restricted mapping.

In mathematics and computer science an arrow often stands for a function: it takes each input to an output. A short vertical stroke across the shaft is the established way to say the function is partial, so some inputs have no output at all. Unicode records this reading for the right-pointing twin ⇸, the partial-function sign of Z notation (a formal language for specifying software precisely).

⇷ is that same arrow mirrored, for writers who draw a mapping from right to left, for example in a diagram where arrows run both ways. Unicode gives ⇷ no note of its own, so a careful text says once what it means by it. Math fonts and broad symbol fonts include it; a font without it shows a box instead.