Há uma explicação detalhada aqui e aqui . Se você estiver realmente interessado, pode ler também o Manual de Normas Debian . Para responder às suas perguntas:
X = esta é a versão debian do pacote
ubuntuY = esta é a versão do Ubuntu do pacote debian
Para citar o Verbatim no primeiro link, package-XubuntuY****” significa:
package = this is the name of the program/library.
X = this is the debian version of the package
if X=0 this means that there is no debian package (or that the ubuntu team has forked a debian package to a newer version than the one found in the debian repositories) _ ex: bzip2-1.0.3-0ubuntu2_ (as shown in this example the debian package might be updated in the meantime and the ubuntu package will probably merged with it on the next version)
ubuntuY**** = this is the Yth ubuntu version of the debian package. if this is missing this mean that it is a clean, unchanged debian package ex: gzip-1.3.5-12 (in this sample, this is the original debian package included in ubuntu) if this is present it means that Ubuntu has taken the debian package and released it with some additional patches or bug fixes. _ ex: sudo-1.6.8p12-1ubuntu6_ (in this sample this is the 6th version of the ubuntu package based on the debian version 1.6.8p12-1 of sudo).