Do you have a link that discusses the 'Matrix' approach in more detail?
Thanks!
The first place I saw it discussed was the 1e AD&D Dungeoneer's Survival Guide. It's described as midway between the fully 'open' (unlinked sandbox) campaign and the linear 'Adventure Path' model. You create an environment, but with lots of links between NPCs, situations and events, with a likely event-trajectory so that even if the PCs go take on the Evil Monks rather than the Dread Necromancer, they still find a clue that the Red Dragon is manipulating both. It's designed to have the advantages of campaign play but still tend to build towards a satisfying campaign climax; though that climax may sometimes be subverted or ignored depending on PC actions and player preference, while still producing a satisfying campaign.