The repeatOffset point is defined for glyphs that are designed to tessellate, such as the wiggly line that follows the symbol, or any of the glyphs in the Multi-segment lines range.
These glyphs are registered such that they may have negative side bearings on either or both the left- and right-hand sides. When entered in a run of text, the advance width produces the correct tessellation. However, in some situations it may not be possible to use a run of text to draw such a line, or the API in use may not provide easy access to the advance width of a glyph (e.g. when using the HTML canvas element).
In these situations, correct tessellation can be achieved by positioning the origin of subsequent glyphs in a tessellating line at the horizontal position defined by the repeatOffset point for a given glyph.
Here, for example, is an illustration of the glyph wiggleTrill:
The vertical dashed lines show the left- and right-hand side bearings for this glyph. The repeatOffset anchor’s coordinates are at the x position of the right-hand side bearing and y = 0. Positioning another trillWiggle glyph at the position of the repeatOffset anchor produces correct tessellation, like this: