What does it mean the number after the stellar class? Magnitude?

[Ignition]

What does it mean the number after the stellar class? Magnitude?

 

[Borek]

Like in G2 for Sun? Subclass. The lower the digit, the colder the star.

 

[Janus]

If instead, you mean the roman numeral sometimes placed after the spectral class, this is the luminosity class as follows:

0 Hyper giant
I Super giant
II Bright giant
III Normal giant
IV Sub giant
V Main sequence (dwarf)
VI Sub dwarf
VII White dwarf

Thus the Sun is a G2 V star.

Sub classes of the luminosity class are labled a,ab & b

 

[Ignition]

Great.