I have never seen a container of motor oil labeled 'Semi-Synthetic"…You are confused because the subject of your thread does not exist…However, “Synthetic Blend” products do exist, more a factor of shelf-space than anything else since NO car maker requires “synthetic blend” oil.
If a car maker has an engine he is worried about, or he wants to extend oil change intervals to 7500 miles and beyond, then they usually specify a specific brand of synthetic oil (100% synthetic).
I see NO benefit in using a “synthetic blend” motor oil over a 100% mineral product…If you feel a synthetic product offers you some benefit, then step up to a 100% synthetic product and get the full benefit!