In the numerical subdivision (0 to 9) within a primary spectral class, what does a lower number indicate?

Answer

A hotter star within that primary class

The classes are subdivided using digits 0 to 9, where a lower number denotes a star that is hotter within its specific primary class; for example, an O0 star is hotter than an O5 star.

In the numerical subdivision (0 to 9) within a primary spectral class, what does a lower number indicate?
physicsastronomytemperaturestarclass