What is the meaning of 6?

The cardinal number six.

A digit in the decimal system of numbering, as well as octal, and hexadecimal.

an indicator that a triad is in first inversion.

the pitch of a tone that is more extreme than 5; it will be higher or lower depending on local convention.

tone number 6, typically identified with light departing yang qu (陽去).

MI6; the agency or a particular agent.

r (= sei, second-person singular)

Source: wiktionary.org