Ligatures - Text

Ligatures are typographic replacement characters for certain letter pairs.

Contextual alternates are alternate characters included in some script typefaces to provide better joining behavior.

A stylistic set is a group of glyph alternates that can be applied to a selected block of text.

Window > Type > OpenType to open the OpenType panel. Now, do one of the following:
Click the Stylistic Sets icon () at the bottom of the panel and choose the desired set.
Select Stylistic Sets from the panel menu and choose the desired set.

SOURCE:
https://helpx.adobe.com/illustrator/using/special-characters.html#use_ligatures_and_contextual_alternates