It's a simple stack that shows raw APIs at the foundation layer, aggregations at the next layer, and applications at the top layer. The left-hand axis uses dollar signs to illustrate that margin (and thus profitability) is most rewarding at the applications layer and most thinly spread at the raw API layer.
But beware: the model is fractal, and therefore all that profitability is relative. The 'A' in API stands for application, and so any top-layer application can become a raw API for someone else's aggregation, driving down its profitability.
I've written more on Spark here and here. Fellow ZDNet blogger Dion Hinchcliffe was also there, along with several other good friends from the SOA blogosphere.