"diakgcn" is the Diablo GCN kernel, it is designed for GPUs built on GCN technology (Radeon HD7000 serries)
CG miner auto-selects the best kernel for your hardware and rarely gets it wrong so just use the default kernel and you'll be fine.
Ah, OK. At one time there was a diagnostic kernel probably started with diag* instead of diak*. Thanks for the correction.
Sam