Difference between regulator's tab and pin?

I think the explanation behind this is interesting as well.

Basically, the exposed tab and the middle leg is generally the same piece of metal.
Basically the tab and middle leg are formed from a single piece of bent metal. Therefore, the tab literally is the middle pin, and you can use it as such. It may even have a (marginally) lower inductance.

_{Image borrowed from ESP}

There is no functional difference between the tab and the pin.

On a lot of SMT versions of these kind of devices the middle pin is cut off and not used - just the tab is used:

Tabbed packages are often used for SMT parts which, despite their small size, have to dissipate up to a couple of Watts. The tab will drain much more heat to the PCB's copper than a pin would.