Earlier quoted context omitted.
It's not the way it is calculated. All payments attached to employment contract are wages, be it regular salary, bonuses, vacation pay or severance. Say the contract defines severance of 1 month for each full year worked capped at 3 months: at the moment contract reaches 1 year anniversary, the employee should be owed 1 month worth of severance upon contract termination
I've worked in the sf tech industry my entire career and my employment contracts have never included guaranteed severance. I've seen severance being paid out to fellow employees but afaik it is done as courtesy and wasn't legally required. Are you European? Maybe it's different there.
Usually severance is consideration for agreeing not to sue the company, not taking trade secrets, etc.