The center tab is a unique critter with a special purpose: Text placed at a center tab in Word 2013 is centered on a line. Unlike centering a paragraph, only text placed at the center tab stop is centered. This feature is ideal for centering text in a header or footer, which is about the only time you need the center tab stop.
The text on the left is at the start of the paragraph, which is left-justified. But the text typed after the tab is centered on the line.
Here's how to make that happen:
Start a new paragraph, one containing text that you want to center.
Center tabs inhabit 1-line paragraphs.
Set a center tab at the 3-inch position on the ruler.
If necessary, show the ruler.
To pluck a center tab stop, click the Tab gizmo on the ruler until a center tab appears (as shown in the margin). Click on the ruler to set the tab stop.
Type some text to start the line (optional).
The text you type should be short; it appears only at the start of the line.
Press the Tab key and type the text to center.
As you type, the text is centered on the line. Don't type too much; remember that the center tab is a single-line thing.
Press Enter to end the line of text.
Obviously, if you want only to center text on a line, centering the entire paragraph is a better choice. Otherwise, this technique finds itself used mostly in page headers and footers.