STS Ocean 5000 is made of pure, pre-streched Dyneema® SK75. Often referred to as "heat-set" Dyneema. The SK75 Dyneema is heated and loaded to a percentage of its breaking load. The process aligns and equalizes the Dyneema fibers which makes for a stronger and lower elongation product.
Stronger than steel, Ocean 5000 STS is also extremely resistant to abrasion and UV degradation.
A relatively stiff line, Ocean 5000 STS is ideal for very highly loaded halyards and stays.
It is often used as emergency back-up rigging — cruisers take note.
Heavy on the technical side...
Dynema® is the Dutch company DSM Dyneema's brand of UHMWPE (Ultra High Molecular Weight PolyEthylene fiber), a long chain, high strength, oriented strand, synthetic fiber with very high strength as well as high abrasion resistance.
This fiber is also known as HMPE (High Modulus PolyEthylene).
Dyneema is available in a number of formulations, SK-60, SK-75, SK-78, SK-90, etc. Each progressively stronger and more expensive.
Of particular interest these days is the difference between Dyneema SK75 and SK78. Dyneema SK78 has the same high strength of SK75, but with lower creep (non-recoverable stretch).
Honeywell International produces a similar UHMWPE fiber, marketed as Spectra®