When the iTunes Store first opened it was pretty simple to understand the rules of “Authorization.” However, now with the App Store, Mac App Store, iTunes Match, Apple TV, etc. it can be really confusing. There are also different rules depending on the type of content in question. For example, if I bought a song back in the day when Apple used Digital Rights Management (DRM) aka “Protected”, then I was limited to only being able to play that song on up to 5 Authorized computers and an unlimited number of iPods. Luckily Apple no longer sells DRM’d tunes, so this problem/limit goes away for any new purchases of “music”. However, other content, such as movies is still DRM protected and therefore still has the same rules enforced.
Authorizing your Mac or PC to play your DRM protected iTunes content
You can have up to five authorized computers (Macintosh, Windows, or both) at a time. To play a purchased item on a sixth computer, you need to deauthorize another one (from that computer). An iPod, iPhone, iPad, or Apple TV doesn’t count as a computer. When you go to play this content on your computer you will be prompted to “Authorize” your computer to play the content. This basically means signing in with the SAME Apple ID that was used to buy the content. Let’s say one of your computer dies or you sold it and forgot to deauthorize it first. You have no way of selectively deauthorizing computers that you no longer have access to from iTunes. The only way to deauthorize those computers is to Deauthorize All of your computer from your iTunes account and then Authorize the ones you want Authorized again from each computer’s iTunes App.
Automatic Downloads and Re-downloading content
Apple now allows you to re-download more than just Apps. You can now re-download music, TV shows and some movies that you’ve purchased in the past. This is great because it means that you can download content directly to a device such as an iPad while you’re out and about without having to return to your computer. However, this comes at a price as you have a restriction on the number of devices that you can “Associate” with an Apple ID:
When you turn on Automatic Downloads or download past purchases on an iOS device or computer, your device or computer will be associated with your Apple ID. Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days. You can view which devices or computers are currently associated, remove unused devices or computers, and see how long before they can be associated with a different Apple ID from the Account Information page in iTunes on your computer:
iTunes Match
I love iTunes match, but it also has limits:
When you add a computer or iOS device to iTunes Match, your computer or device will be associated to the Apple ID being used for iTunes Match. Your Apple ID can have up to 10 devices and computers (combined) associated with it. This means computers, iOS devices and Apple TVs. That means it will be a lot easier to hit this limit since it includes everything!
iTunes Match is limited to 25,000 songs.
iTunes Store purchases made with the same Apple ID being used for iTunes Match do not count towards the 25,000 song limit.
Unmatched content will be uploaded as is; upload time varies depending on the amounts uploaded and local network speeds.
Song files over 200 MB will not be uploaded to iCloud.
Songs containing DRM (Digital Rights Management) will not be matched or uploaded to iCloud unless your computer is authorized for playback of that content.
iOS Apps
Luckily the policy is a little more generous when it comes to Apps. There is no limit on the number of iOS devices that you can install the Apps you associated with your Apple ID. Also since the very beginning Apple allows you to delete and re-download iOS Apps. In other words once you own it, you own it for life. You can always re-download an App that you deleted or got wiped out due to a crash.
The Bottom Line
Luckily none of the above is a concern for your own content, content you ripped from CDs or movies from DVDs or iTunes Plus content. However, if you bought Protected content from the iTunes Store then you’ll have to play by the ever changing rules above. In Apple’s defense these rules were more than likely brought to us by the authors, labels, movie studios, etc. Heaven forbid that you have more than 5 computers or 10 devices! 
