Sales here means promotions: a percentage off part of the catalogue for a period, set up under Point of Sale > Sales.
You can run any number of them at once. When two of them cover the same product they compound rather than add: 10 percent off the whole store plus 20 percent off one brand leaves 28 percent off that brand, not 30. The store-wide sale comes off first, and the narrower one comes off what is left.
The order is always broadest first, narrowest last. Manifold works out which is narrower by how many of the two lists a sale constrains, then how many values it names, then how much it carves out in its Except section.
**When you do not want them to stack**
Tick "Do not combine this sale with other sales" on the sale. It then never compounds. Where it meets another sale on the same product, the customer gets the better single outcome: that sale on its own, or everything still willing to stack, compounded among itself. With the usual two sales that is simply the bigger discount winning.
Use it when you are advertising a fixed number. A brand sale promoted at 20 percent off should not quietly become 28 percent because a store-wide 10 is also running.
**Seeing it before a customer does**
Open a sale and click What It Covers. Where another sale reaches the same product, the row names it and shows what the customer would actually pay. A row marked "best one only" is one where a sale set not to combine is involved, so only the better of the two applied. A struck-through price in the On sale column means another sale wins that product outright.