A campaign is a container for a promotion: a name, a time window, and a targeting scope. Use it to answer one question — did this promotion drive extra sales, and at what margin cost? Marketing → Campaigns lists them and their status.
A campaign does notneed a discount. If you run a single-store promotion that the POS can't discount on its own (DK applies discounts to every till at once), create a campaign scoped to that store and category anyway — you still get the sales measurement.
The campaign page shows two scorecards over the campaign window, each compared to a baseline:
Switch the baseline with the Compare against chips: the prior period (same length, immediately before), the same period last month (day-of-month aligned, so June 1 compares to May 1), the same period last year, or another campaign you picked.
When you apply a discount on Marketing → Discounts, pick a campaign in the Campaigndropdown. Linked discounts appear on the campaign page and feed the "discount given up" estimate. Discounts created before campaigns existed keep their old text label and still group together.