Font sizes are nominally indicated at points where 1 point = 1/72 of an inch. This is true not only in IB and Xcode, but also in almost all areas. Whenever you specify the font size in a MacOS (X or classic) application, including Photoshop, units are dots. I would suggest that this is true on Windows as well.
Remember that a type that uses the same font and size may be displayed differently on different displays depending on the resolution of the display, so “1 dot” may not correspond to 1/72 of an inch on the screen. Early Mac computers showed that they were very close to 72dpi, so they were very WYSIWYG .