Let's talk about severance, pip, and lay-offs from HR's perspective.
First if you have been picked into the group to be reduced. You usually will be given two options as mentioned before: A voluntary resignation (severance) package and pip. The voluntary severance package is not actually "voluntary" since not everyone can request it for leaving, comparing to some other companies which openly offers to encourage people to leave. The company uses this package to exchange your legal right for suing WT which can costs up to hundreds of thousand dollars plus lawyer fees. It is not easy to win the legal battle but it is what the hr guys don't want to see. Taking voluntary package will generally makes you not eligible to receive the unemployment benefit as you resign. However severance pay as heard is 3 + 0.5 * years of service, for ex if you worked for 6 yrs, you get 6 months which is better than the unemployment benefit and many people choice this and move on.
If your choice is not to take the package, you will join pip which is used to give you some time to find another job and stress you to give up by walking-away or take the package. The pip itself is very stressful and risky to both the company and employees. There are many factors includes legal basis, mental overloading, and even physical conditions making pip process notorious. For some who already make up their minds, in the end of pip, they will terminate (fire) you. However, you might be eligible for the unemployment benefit in this case. If the company contests your benefit, they have to prove in the court that you construct "misconduct" which makes the company suffers lose. This is also a burden and risk for the company to commit it into legal way. In the end of pip, you also will reserve your legal right for WT of which most people don't use it.
Hopefully it makes it clear for the thing that have been discussed so long.