Looks very much like Amsteel, a hollow 12-plait, although Amsteel is rounded and this line appears flattened.
Used a bunch of Amsteel recently re-rigging my sailboat. A great substitute and improvement over wire rope for many applications. Extremely strong, very low stretch and a snap to splice.
Amsteel is not something that works well as a general purpose line. It is very expensive and has essentially no stretch or shock absorption capacity. It is a high-tech substitute for flexible wire rope.
