Fast acting calcifediol is used in the more reliable Spanish, RCT vitamin D studies, instead of very slow absorbing and acting cholecalciferol (vitamin D3), which is fine for everyday use (5,000-10,000 iu/day), but not when a person needs a lot fast — has low levels and Covid cytokine storm lung issues.

Is calcifediol better than cholecalciferol for vitamin D supplementation?

https://pubmed.ncbi.nlm.nih.gov/29713796