I'm a little uncertain of what you mean by "tab", but the lower button on the shifter is the overdrive on/off button, making that a 4 speed auto, not a 3 speed as your photo is labeled (or at least a 4 speed shifter) If your talking about the little "plug" just in front of the "P", it's to access the override release for the brake interlock that won't allow a shift out of park when the brake pedal is not depressed. If the system were to fail, you could remove the plug, push the release and shift out of park.
If you're talking about something else altogether, I appologise for my long-winded-ness

, and ask you to clarify.