the prefix number of the configuration filename

[Dogz]

Hi all,

I am very curious about the prefix number of the configuration
filename, such as 10-auth.conf
10-director.conf
10-logging.conf
10-mail.conf
10-master.conf
10-ssl.conf
15-lda.conf
15-mailboxes.conf
20-imap.conf
20-lmtp.conf
20-managesieve.conf
20-pop3.conf
90-acl.conf
90-plugin.conf
90-quota.conf
90-sieve.conf
90-sieve-extprograms.conf

What are those meanings of 10,15,20 and 90 ?

Please advise me and thank you in advance.

-- 
Best Regards,
Dogz