There are five real forms of the exceptional Lie group, $E_6$. Four of them are notated as:

• The split form as EI or $E_{6(6)}$

• The quasi-split form as EII or $E_{6(2)}$

• EIII or $E_{6(-14)}$

• EIV or $E_{6(-26)}$

What do the annotations to $E_6$ actually indicate and are they also used for real forms for Lie groups in general?