Just worded in a nice way. Its a reputation saving measure. Those asked to resign retain their rank and benefits and allow for a controlled exit. Being forced out and relieved of command, they are forcefully removed, could face, but not always, an investigation, that could result in demotion and loss of rank and benefits.

Others have already described the normal situation -- asked to resign is fired but with rank and pension and a fig leaf that you left voluntarily.

In Donnie's case, it also very likely means he made you sign a nondisclosure agreement that includes a non disparagement clause as well as an agreement not to sue. That's not entirely uncommon normally, but I'd be shocked if Donnie passed up the opportunity to further control and suppress someone he screwed over.