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If you load http://pomax.github.io/CFF-glyphlet-fonts in Firefox, you'll note that the "small" font, without GSUB table, renders the character "~" as a rectangle with a cutout. The second font has a GSUB table for the 'liga' feature with a lookup type 4 to turn the sequence "c,u,s,t,o,m" into "~", http://github.com/Pomax/CFF-glyphlet-fonts/blob/gh-pages/binaries/with%20GSUB/customfont.gsub.ttx#L271, which works in Firefox, but not in Chrome or IE (of course IE doesn't like plain .otf, so the fonts are repackaged as uncompressed WOFF as well, as fallback option).
For which reason and when are 'subtables' created? Adobe's OpenType Feature File Specification does not help me any further understanding the structure of GPOS/GSUB.