Within a given folder, you can set one sample set as "active". (To set the active sample set: select the target sample set, and click Make Active.) Only one sample set can be "active" within a given folder.

The active sample set adds its columns to the system table exp.Materials, giving you some control over the columns in that system table. Individual records you import to a sample set are copied to the exp.Materials table, forming a complete list of all individual samples in the current folder, making it a convenient place to link to, or lookup to, when building sample-related queries. Adding columns from the "active" sample set is a way to show custom properties in data grids when the server doesn't know up front that it will be pointed at a specific sample set. If you have multiple sample sets in a single folder, you can't pull in all of their properties, only the ones from the active sample set.