iTunes Match is primarily a library syncing service that allows you to have the same library of content and playlists across multiple computers and devices. If you no longer need that service you can cancel it. You can use the iCloud Download and iCloud Status columns in the Songs view of the library to review what it is in it, and where it comes from.
A smart playlist like this will show any of your music and other media purchases that are currently referenced in the cloud only and not stored on your computer.
If you terminate your iTunes Match subscription you lose access to any matched or uploaded music that is stored only in the cloud. Your purchases should remain available, although there is still a small risk that some items might be removed from the store, with your redownload rights expiring as a result. Ideally you should have locally stored downloads of all media, which you also have backed up to a secondary drive.
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