 Right side bearing

The right side bearing measures the signed gap from the right edge of a glyph's ink bounding box to the point specified by its default horizontal glyph advance. If the origin is $x=0$, the advance is $a$, and the ink edges are $x_L,x_R$, thenThus the advance splits into a left side bearing, an ink width and a right side bearing. OpenType uses this relation to derive the right side bearing from the horizontal metrics and outline bounds.^[ref]

For the hypothetical ink interval $40\le x\le 540$ and advance 600 font units, the right side bearing is 60 font units. If a different glyph extends to $x_R=650$ with the same advance 600, its right side bearing is $-50$ font units: ink crosses the next nominal origin. A negative bearing is therefore meaningful rather than an invalid width.

These are metrics of an individual default glyph. Kerning and other glyph positioning rules can change the actual gap between adjacent shapes, so the right side bearing alone does not determine the visual spacing of a whole run.