The “single ended” (SE) vessels only open on one end (one cap). The “double ended” (DE) sample vessels have vessel covers on both ends and can be opened on both ends. The smaller vessels are double ended to make it easier to clean inside the vessel. The larger vessels have a sufficient diameter, so opening only one end is acceptable. From a costing standpoint, once you get above 100 ml in sample vessel size, the cost savings for a single ended vessel is significant versus a double ended vessel.