Easy access to important information keeps migrating out from the depths of the OS X System Information utility up to easy access from the About This Mac window. However, some of that information presented in a small graph in a small window requires a good bit of context to understand, and the footnotes are missing.
Inattentive longtime Mac users may be surprised to see a row of tabs in the About This Mac window (I believe these arrived in Mac OS X Lion). Along with the longstanding System Report... and Software Update... buttons at the bottom of the window are topside tabs for Overview, Displays, Storage, Memory, Support and Service.
The Storage tab appears straightforward: there's the traditional list of storage devices attached to the system and then a graph showing the capacity of each drive, the free capacity and a colored breakdown of the types of files on the drive into Audio, Movies, Photos, Apps, Backups and Other categories. It's the Other files that bring the questions. What exactly is Other?
In the screenshot above, you can see that my MacBook Pro has an SSD as the topmost boot volume and then a 750GB hard drive divided into two volumes: one is a bootable system and the other, larger one holds my iTunes library. The optical drive is external.
A recently revised Apple Support Note describes the Other category:
Sometimes the "Other" category in the About This Mac window may seem large. About This Mac uses this category to count any files that aren't recognized as one of the other listed file types (audio, movie, photos, apps, backups). Examples of files that may be calculated as "other" include:
Items in the OS X folders such as the System folder and caches
Personal information such as documents, contacts, calendar data
App Plugins or extensions
Media files that cannot be classified by Spotlight as a media file because they are located inside of a package
File types not recognized by Spotlight
James Pond at his Pondini OS X and Time Machine Tips blog, points out that what comprises the Backups category is a bit complicated. On the a drive backed up by Time Machine there will be snapshots, which are temporary and will be deleted automatically if necessary by the system. Pond points out that the Storage tab, and Get Info and the Finder treat these calculations differently — for the latter two results, these snapshots are considered free space.
Backups by third-party programs are Other files.
I am used to looking at Get Info for a quick view of the unused capacity of a volume. However, now I am unsure when the System will decide to remove stored Local Snapshots, so perhaps the Storage tab's figure may be a better measure of usable capacity at any given moment. If you're talking to a client about free capacity, it might be best to know where that figure was found: the Storage tab or Get Info/Finder.
In addition, since the Storage tab view uses Spotlight, it's dependent on the accuracy of its index. If there's been a lot of disk activity and Spotlight's indexes haven't been updated, then the Other result may be inaccurate.
For a more accurate, granular-level understanding of capacity on a drive, there are a number of utilities for mapping files and applications. I use Erwin Bonsma's Grand Perspective, which can show the sizes of content files and libraries as well as the individual components of applications. It's very colorful.
For example, in the screenshot above, you can see the long aqua box on the right side is the Dragon speech-recognition engine's English data. I had selected a label template for Pages from a third-party vendor and you can see its path in the info box.