Confused of build number schema

[Markus KARG <ka...@quipsy.de>]

In "BetterBuildsWithMaven.pdf" I have read the following:

"In the current version scheme, the snapshot (as shown above)
is a special case where the qualifier and build number are both allowed. 
In a regular version, you can
provide only the qualifier, or only the build number. It is intended 
that the qualifier indicates a version
prior to release (for example, alpha-1, beta-1, rc1), while the build 
number is an increment after
release to indicate patched builds."

In fact I am confused now. The first sentence tells that "-1" is only 
allowed with "-SNAPSHOT". The second sentence shows an example where 
"-1" is used together with "alpha". But actually "alpha" is not 
"SNAPSHOT". So the example is forbidden. Strange. Confusing.

Later there is a sentence telling that "1.2-beta-1 is newer than 
1.2-beta". So actually "-1" seems to be allowed not only with "SNAPSHOT" 
but also with "beta"...?!

Can somebody explain to me the deeper logic of that? I mean, either "-1" 
os only allowed with "SNAPSHOT", or it is not. OR: "beta" is TREATED 
like SNAPSHOT?

I do not understand this.
Please explain...

Thanks
Markus