What Zoe said, plus I agree that one usually uses insulin to "correct" high blood glucose and one "treats" a low with glucose/food.
For example, if I wake up in the morning and my before-breakfast BG reading is 160 mg/dl, I prepare a single injection of Novolog which contains both a "bolus" for my breakfast carbs and a "correction" for the 160.
Please note: This is MY correction, do not copy it! I'm an insulin-resistant T2 so my insulin doses will be much higher than a T1's insulin doses, as a general rule.
Anyway, I do my correction by subtracting 100 (my target blood glucose) from 160, which leaves me with 60 mg/dl to correct, and then divide 60 by 8 (my correction factor is one unit of Novolog for each eight points mg/dl above my target of 100 mg/dl) which gives me a correction dose of 7.5 IU of Novolog.
If I'm having 15 grams of carbs for my breakfast, I divide that by 4 (my insulin:carb ratio is 1:4) which is 3.75 IU. I add together 7.5 (correction) and 3.75 (bolus) and get 11.25 IU total Novolog, which I will round up to 12 IU because I'm so very insulin resistant -- it would be OK to dip into the 90's anyway.
So that's how I calculate my insulin if I'm going to combine a bolus and a correction.
But what if it's been four hours since lunch, dinner in an unfamiliar restaurant won't be for an hour or two, and I test my BG and it's a bit high, say 180? If I know that I don't have any more Novolog on board (after four hours), I can just calculate a correction:
180 mg/dl - 100 mg/dl = 80 mg/dl
80 mg/dl / 8 (correction factor) = 10 IU of Novolog
I don't HAVE to eat yet, I can just go ahead and correct the hyperglycemia and wait until I know what I'm having for dinner and then, at the restaurant, bolus JUST for the meal when the time comes, say two hours after my correction.
What I would NOT want to do two hours after a correction is test again, and correct again because the Novolog from the previous correction is still being metabolized and if I corrected again I would "stack" the corrections and probably go hypo.
Clear as mud? ;o)