安装 deb 包的过程求助

linuxier

最近在看 debian policy manual, 里面讲到安装和升级软件包的过程时有点不懂，

The procedure on installation/upgrade/overwrite/disappear (i.e., when running dpkg --unpack, or the unpack stage of dpkg --install) is as follows. In each case, if a major error occurs (unless listed below) the actions are, in general, run backwards - this means that the maintainer scripts are run with different arguments in reverse order. These are the "error unwind" calls listed below.

这里的第二句是什么意思？什么是 "error unwind"？

1.      If a version of the package is already installed, call

           old-prerm upgrade new-version

上面这句是什么意思，对于已经安装软件包的升级照说是按某个顺序运行 deb 包里的脚本，不懂上面这句。