I haven't run into this in person, but my dad worked for years at a BigCo engineering firm that had multiple rounds of layoffs, and they seemed to use a fourth strategy: try to get as many volunteers as possible by offering some severance packages, and just take whatever random assortment of people said yes. The theory was that that was the way to reduce headcount with the least impact on morale, since nobody was actually being fired. In practice some people were strongly hinted that it was in their best interests to take the package, though. It also tended to be de-facto biased towards getting rid of the older/more-expensive employees (who would just take early retirement) without overtly singling them out.