what is STRUCTURAL,AUXILIARY, ABSTRACT,top?

[<zzhhyyyy@sohu.com>]

I don't understand what STRUCTURAL,AUXILIARY, ABSTRACT mean,For example:
( 1.3.6.1.4.1.1466.344 NAME 'dcObject' SUP top AUXILIARY MUST dc )
"AUXILIARY" in the object class definition above may become STRUCTURAL,ABSTRACT in other object class definition,why ?

furthermore,i don't know what object class "top" mean and how to use it.

I still have another problem,for example:
dn: dc=critical-angle,dc=com
  objectClass: top
  objectClass: organization
  objectClass: dcObject
  dc: critical-angle
  o: Critical Angle Inc.

( 0.9.2342.19200300.100.4.13 NAME 'domain' SUP top STRUCTURAL
MUST dc)
dn: dc=tcp,dc=critical-angle,dc=com
  objectClass: top
  objectClass: domain
  dc: tcp
  description: a placeholder entry used with SRV records
I think that the position of dn:dc=tcp,dc=critical-angle,dc=com is under dn: dc=critical-angle,dc=com,so dn:dc=tcp,dc=critical-angle,dc=com should be rewrited in the following:
dn: dc=tcp,dc=critical-angle,dc=com
  objectClass: top
  objectclass: organization
  objectClass: domain
  dc: tcp
  description: a placeholder entry used with SRV records

My idea is right? please help me!