Half switched outlet, has both whites connected
Here's  the thing about the tab. 

Each side of the receptacle has two possible modes.  


If you remove the tab, you isolate that side (hot or neutral) of the upper and lower socket. Now you have separate control of the two sockets, but, you must connect a separate wire to each half.  
If you keep the tab, both upper and lower sockets are connected.  There are still two screws, but they are connected. You can supply both sockets from 1 wire on that side.   Or, you can use the two screws as a splice block: Attach two wires, those two wires are connected to each other and also to both sockets on that side.  It is a very handy way to continue power onward to the next socket.  


You can only use one mode, and when you snap the tab off, you're committed!  And with tab broken off, each screw can take only one wire (except for some Leviton and other brands whose $3 commercial tier sockets allow 2 wires under every screw.). Never put one wire on a screw and another on the backstab. 

The splicing method isn't required, you can accomplish the same thing by adding a short pigtail to one receptacle screw and joining it to the other two wires with a wire nut.  In fact, in some conditions, the splicing method is forbidden and you have to use a pigtail.  For instance, the "must pigtail" rule applies to all grounds. 

I myself use the pigtail method almost 100%, because I am usually working on ladder/knees/cramped location in a stress position, and wire nutting 3 groups of wires together is much faster than putting 5 wires onto screws.