O Manual de Políticas Debian tem a dizer sobre o campo de versão, que responde algumas partes da sua pergunta:
Format
The format is:
[epoch:]upstream_version[-debian_revision]The three components here are:
epoch
This is a single (generally small) unsigned integer. It may be omitted, in which case zero is assumed. If it is omitted then the upstream_version may not contain any colons. It is provided to allow mistakes in the version numbers of older versions of a package, and also a package's previous version numbering schemes, to be left behind.
upstream_version
This is the main part of the version number. It is usually the version number of the original ("upstream") package from which the .deb file has been made, if this is applicable. Usually this will be in the same format as that specified by the upstream author(s); however, it may need to be reformatted to fit into the package management system's format and comparison scheme.
The comparison behavior of the package management system with respect to the upstream_version is described below. The upstream_version portion of the version number is mandatory.
The upstream_version may contain only alphanumerics[36] and the characters
"."(full stop),"+"(plus),"-"(hyphen),":"(colon),"~"(tilde) and should start with a digit. If there is no debian_revision then hyphens are not allowed; if there is no epoch then colons are not allowed.debian_revision
This part of the version number specifies the version of the Debian package based on the upstream version. It may contain only alphanumerics and the characters
"."(full stop),"+"(plus),"~"(tilde) and is compared in the same way as the upstream_version is.It is optional; if it isn't present then the upstream_version may not contain a hyphen. This format represents the case where a piece of software was written specifically to be a Debian package, where the Debian package source must always be identical to the pristine source and therefore no revision indication is required.
It is conventional to restart the debian_revision at 1 each time the upstream_version is increased.
The package management system will break the version number apart at the last hyphen in the string (if there is one) to determine the upstream_version and debian_revision. The absence of a debian_revision is equivalent to a debian_revision of 0.
Comparison
When comparing two version numbers, first the epoch of each are compared, then the upstream_version if epoch is equal, and then debian_revision if upstream_version is also equal. epoch is compared numerically. The upstream_version and debian_revision parts are compared by the package management system using the following algorithm:
The strings are compared from left to right.
First the initial part of each string consisting entirely of non-digit characters is determined. These two parts (one of which may be empty) are compared lexically. If a difference is found it is returned. The lexical comparison is a comparison of ASCII values modified so that all the letters sort earlier than all the non-letters and so that a tilde sorts before anything, even the end of a part. For example, the following parts are in sorted order from earliest to latest:
~~,~~a,~, the empty part,a.Then the initial part of the remainder of each string which consists entirely of digit characters is determined. The numerical values of these two parts are compared, and any difference found is returned as the result of the comparison. For these purposes an empty string (which can only occur at the end of one or both version strings being compared) counts as zero.
These two steps (comparing and removing initial non-digit strings and initial digit strings) are repeated until a difference is found or both strings are exhausted.
Note that the purpose of epochs is to allow us to leave behind mistakes in version numbering, and to cope with situations where the version numbering scheme changes. It is not intended to cope with version numbers containing strings of letters which the package management system cannot interpret (such as
ALPHAorpre-), or with silly orderings.
ubuntu indicará que o pacote foi criado especificamente para o Ubuntu. As sequências alpha e build não parecem ter nenhum significado específico, mas dfsg refere-se a um pacote que foi modificado para conformidade com as Normas para o Software Livre Debian .