It is an outdated method of determining insulin dose, but saying "if your blood sugar is between this number and that number take this many units". Insulin to carb ratio has replaced that as the best way to dose for meals.
Good explanation, Terry. Yep, it amazes me how people think we could use the same amount of insulin for a salad as for a plate of pasta. I kind of assumed the logic of carbs causing blood sugar spikes if not accompanied by sufficient insulin pretty basic and understood by the medical profession. But I've mentioned on here before having a friend who is an RN whose knowledge and experience I've always found superlative. I mentioned limiting carbs and she said, "I've heard of low carb for weight loss, but do you mean for Diabetes". Well....duh!
I think it may also be a relic of a time when people thought patients were too dumb to estimate carbs , divide by an I:C ratio, calculate a corection dose with their Insulin Sensitivity Factor, add them and inject. Instead they give the patient a simple table.
And the control is suboptimal.
In japan when i was DX at three and going into my late teen we used the slide scale before carbs counting really came in,In UK use a brilliant system called Daphne and my sister partner went on a course i think lasted a week and in compassed everything to learn about crab counting and insulin ratio.