Comment 18 for bug 1145560

The ‘+’ may not be standard, but it’s necessary for openafs, for the following subtle reason.

If you create a binary module package with
  apt-get install module-assistant openafs-modules-source
  m-a build openafs
then the resulting package is versioned as
  openafs-modules-3.2.0-38-generic_1.6.1-1+3.2.0-38.61_amd64.deb
(A site with many OpenAFS client systems might throw this binary module package into an apt repository so that it doesn’t need to be rebuilt on every system.)

Now if you were to upgrade from 1.6.1-1 to 1.6.1-1ubuntu0.1 and do the same thing, you’d get
  openafs-modules-3.2.0-38-generic_1.6.1-1ubuntu0.1+3.2.0-38.61_amd64.deb
As you can check with dpkg --compare-versions, this is a _smaller_ version number!

That’s why the openafs package has been getting updates versioned like 1.6.1-1+ubuntu0.1 instead of 1.6.1-1ubuntu0.1, at least when someone has been around to point this problem out.