\octave check: first check, second check

[Graham Percival]

Hi guys, your friendly Doc Editor here again.  Can somebody other than 
Han, Jan,
or Mats answer this?   (not that I have anything against you guys; I 
just don't want
to bother you with this and get more people involved in writing the 
manual.  :)

Could somebody explain this part of the manual?

---    (from 5.2.2, Octave check)
There is also a syntax that is separate from the notes. The syntax
     \octave pitch


 This checks that pitch (without quotes) yields pitch (with quotes) in 
\relative mode. If not, a warning is printed, and the octave is 
corrected, for example, the first check is passed successfully. The 
second check fails with an error message. The octave is adjusted so the 
following notes are in the correct octave once again.
     \relative c' {
       e
       \octave a'
       \octave b'
     }
---

Specifically, what's the "first check", "second check" business?  I 
don't use
\octave, and I can't understand the second paragraph, so I'm hoping
somebody can clarify this.

Cheers,
- Graham