Markup and margin are the single most commonly confused pair of numbers in pricing, because they describe the exact same dollar amount of profit from two different starting points. If something costs you $60 and you sell it for $100, your profit is $40. As a markup (profit ÷ cost), that's 66.7%. As a margin (profit ÷ selling price), that's 40%. Both are correct; they're just answering different questions.
The practical risk is using a markup percentage where a margin percentage was intended, or vice versa — a common mistake is setting a "50% margin" by simply adding 50% to cost, which actually produces only a 33.3% margin, not 50%. If you want a true 50% margin, you need a 100% markup on cost.
As a rule of thumb: markup is always a larger number than margin for the same sale, because cost is always lower than selling price, so the same profit dollars make up a bigger share of the smaller number.