A combo steel is most commonly used for custom meat fabrication (boutique butchers). The Fine edge is mildly abrasive compared to a regular cut steel but more aggressive than a polished cut steel. This side will help realign the edge if you hit a bone while sharpening. The polished edge is not abrasive at all. Its sole purpose is to move the edge if it becomes bent. The polished edge will not ware down the knife as fast.
If you are not familiar with this type of sharpening steel you most likely need a regular cut steel. The combo steels are for very specific applications.