D3 is the kind that you wish to supplement with (assuming that you have decided to do so); it is much more absorbable and, in almost all cases, when someone is talking about or selling “Vitamin D,” they are talking about or selling D3.
Note that the absorption of D3 is enhanced when you take it with fats (it is fat-soluble). Typically, D3 comes in a capsule that contains some fat, but not very much. I tend to take mine with my fish oil and vitamin K supplement in the evening, after a dinner with some fats in it...plus the fish oil IS fat and the K comes like the D3 in a capsule that contains fat.
Thank you for the info. I’ve been taking D3 for some months now, but used to take D. I’ve been unusually healthy most of my 87 years because I take most of the best vitamins, especially Vitamin C+ Rose hips.