EN: Phantom points
In a truetype font there are a few points added to each glyph which are not specified by the contours that make up the glyph. These are called phantom points. One of these points represents the left side bearing, and the other the advance width of the glyph. Truetype instructions (hints) are allowed to move these points around just as any other points may be moved -- thus changing the left-side-bearing or the advance width. Early versions of TrueType supplied just these two phantoms, more recent versions also supply a phantom for the top sidebearing and a phantom for the vertical advance width.
Font Forge Glossary