You should aim to have 15-20% of your total internal storage as free space (~30-40 GB as an absolute minimum). If you do not have enough free space on your Mac’s internal storage, then here are some steps to take to safely free up space:
Free up storage space on Mac - Apple Support
Optimize storage space on your Mac - Apple Support
How to free up ‘System Data’ and other st… - Apple Community
"System Data" comprises your data, but they are typically hidden files that Spotlight cannot assign to a specific category like Documents or Applications. Examples include Outlook attachments, iOS updates, Cloud storage caches, etc. Starting with macOS 27 Golden Gate, System Data also includes what macOS used to call ‘purgeable’ storage – system caches, Time Machine snapshots and some other data, though the user had no control over the purging. Trying to manually clean out hidden files in System Data is likely to cause more problems than it solves, macOS hides those files for a good reason. Follow the steps in the linked articles above.
You can check the amount of free space you have on your Mac in a variety of ways:
- Select your internal drive then click File menu > Get Info, check Available space
- Go to System Settings > General > Storage, check the graphical display at the top of the settings pane
- In Disk Utility (in Applications > Utilities), by selecting the main container disk below the drive itself (you may need to select View menu > Show all devices to see it)
- In the System Report – hold the Option key and click the Apple menu > System Information (the top menu item), look for the name of your internal drive (Macintosh HD, unless you have renamed it as I did)
On Tahoe, use one of the last two options, the first two show 'available' space which is not the same as free.

More detail on this change in this User Tip:
System Data and Free Space Changes in Gol… - Apple Community
The best advice is to just ignore the System Data category. ‘System Data’ is merely a catch-all category for files that Spotlight cannot assign to specific categories like Applications, Documents, Photos, etc. In fact, what is now shown as System Data used to be reported as ‘Other’, Apple changed the label in macOS 12 Monterey. System Data probably sounds better, but Other was closer to reality.