Spacing classes provides a way to automatically copy kerning, lsb (left sidebearing) and rsb (right sidebearing) from one character to another in your font. Menu → Spacing and Kerning → Spacing Classes.
You can also use spacing classes to create a mono spaced font.The absolute setting will place the the left and right line at the same coordinate for all glyphs that your spacing class updates. The absolute spacing class copies all spacing properties between the two letters of the class. Since a = b will all kerning pairs be copied from 'a' to 'b' and vice versa.