In OpenType fonts, mark attachment is the GPOS (glyph positioning) rule which dynamically positions diacritical marks (accents and the like) relative to base glyphs or other marks. The rules for positioning marks relative to base glyphs are defined in the “mark” (mark to base) feature. The rules for positioning marks relative to other marks are defined in the “mkmk” (mark to mark) feature.
FontLab can automatically generate these two features using the Anchors set in base and accent glyphs. See the Anchors and Pins and Features panel sections for details.