I have two bikes, one with Campagnolo and one with SRAM red. I want to be able to swap wheels (Primary for Triathlons) but also on my trainer.
Why do I need to be aware of?
In theory, you would want to make sure that the cog-cog distance for each cassette is preferably equal, or at least close enough. You may need to adjust your limit screws as well when changing wheels. Campagnolo maintains variable spacing between its cogs, whereas neither SRAM nor Shimano do. Ergo, while the average cog-cog distance is identical, the actual distance between cogs is slightly different on Campagnolo cassettes.
However, these differences are not of practical significance. 11s cassettes are functionally interchangeable across all 3 manufacturers. This is widely known.
Some riders report that they prefer to keep a like chain or a 'neutral' chain (as opposed to a competitor's chain) with a like cassette to maximize shifting performance. For example, with a Campagnolo cassette on a Shimano drivetrain, riders might prefer a Campagnolo chain, or if not then a KMC or Yaban chain. However, I'm not certain how much of this is a placebo effect versus an actual shifting difference.
12s cassettes from the big 3 groups should be interchangeable as well, at least in terms of cog-cog distance. There simply isn't enough space left for them to vary in that dimension. If you are planning to interchange cassette types on a 12s system, note that SRAM's AXS 12s chain may only be officially compatible with SRAM cogs and chainrings, because that chain has slightly larger rollers specified. I believe I have heard some riders remark that this is, again, not a practical concern, but I haven't kept track of this issue.