'ss01' gives ⟨a⟩ a single-story glyph. All derivatives of ⟨a⟩ are affected too, except U+00AA FEMININE ORDINAL INDICATOR and U+0201 LATIN SMALL LETTER A WITH DOUBLE GRAVE. (It also skips ⟨æ⟩ but I assume that was intentional to avoid confusion with ⟨œ⟩.)
'ss05' adds serifs to ⟨I⟩ and its derivatives, except U+0208 LATIN CAPITAL LETTER I WITH DOUBLE GRAVE, U+020A LATIN CAPITAL LETTER I WITH INVERTED BREVE, and U+1E2E LATIN CAPITAL LETTER I WITH DIAERESIS AND ACUTE. (It also skips ⟨IJ⟩ but I assume that was intentional, since 'ss04' applies to that.)
⟨B⟩ and its derivatives used to be sans-serif by default in previous versions, but now they have serifs by default, unless 'ss04' is enabled. The exceptions are U+0E3F THAI CURRENCY SYMBOL BAHT and U+20BF BITCOIN SIGN, which have no glyphs with serifs. Is that intentional?
Some stylistic sets are incomplete.
'ss01' gives ⟨a⟩ a single-story glyph. All derivatives of ⟨a⟩ are affected too, except U+00AA FEMININE ORDINAL INDICATOR and U+0201 LATIN SMALL LETTER A WITH DOUBLE GRAVE. (It also skips ⟨æ⟩ but I assume that was intentional to avoid confusion with ⟨œ⟩.)
'ss05' adds serifs to ⟨I⟩ and its derivatives, except U+0208 LATIN CAPITAL LETTER I WITH DOUBLE GRAVE, U+020A LATIN CAPITAL LETTER I WITH INVERTED BREVE, and U+1E2E LATIN CAPITAL LETTER I WITH DIAERESIS AND ACUTE. (It also skips ⟨IJ⟩ but I assume that was intentional, since 'ss04' applies to that.)
⟨B⟩ and its derivatives used to be sans-serif by default in previous versions, but now they have serifs by default, unless 'ss04' is enabled. The exceptions are U+0E3F THAI CURRENCY SYMBOL BAHT and U+20BF BITCOIN SIGN, which have no glyphs with serifs. Is that intentional?