This common activities option will create an outline custom instance list browser like this example:
The outline created looks like the shipped simple grid style, so it has been watermarked to remind you that it is not that.
It is recommended that you do not create custom instance list browsers that are like shipped versions because that means you must reproduce most of the built-in logic yourself. That includes the built-in logic available now and in the future.
Instead reserve the creation of custom instance list browsers for situations where the visualization of the instance list entries is radically different to the available shipped versions.
For example, an instance presented as a calendar:
Or as a carousel: