The OpenType Layout common table formats, shared by GPOS and GSUB.

Both tables have the same outer structure - a script list, a feature list and a lookup list - and both reach glyphs through coverage tables. Only the innermost subtables differ: GSUB substitutes glyphs, GPOS positions them. That shared outer structure is what lives here.

The indirection is deep because the format is: a script tag selects a language system, which selects feature indices, which select lookup indices, which select lookup subtables. A parser that wants "the kerning pairs" has to walk all five levels.