What is system data and why I can't clean it

System Data is where the User Account is located

If more than 1 user, Users Accounts

Time Machine Backups would also be part of System Data

From another contributor @etresoft regarding Free Space and Available Space 

Free vs available disk space huge differe… - Apple Community

Quote >>  “ The "available" storage is the amount of used storage that the operating system could automatically delete if it felt that it was really necessary. The "free" storage is the amount that you can actually use for something.

There are system processes that run in the background and automatically delete some of the "available" storage and convert it to "free". If you completely run out of storage, then those system processes will try a little harder. When you "delete" files you are just hinting to the operating system that you don't need those files anymore. The operating system will eventually remove them, but on its own schedule.

Certain tools will allow you to force the issue and manually clean up some of this storage and manually delete local snapshots. But that is only temporary. "  << End Quote 

Purgeable Space is controlled by the operating system. When the system needs more empty space, it moves some purgeable space to empty space.

How to delete Time Machine snapshots on your Mac.

This often occurs if the Time Machine Drive isn’t attached to the computer and TM Backup is set to run on a schedule.

TM Backup makes snapshots on the internal drive until the Time Machine Drive is attached.

Then, the snapshots are transferred to the external drive.

View APFS snapshots in Disk Utility on Mac

System Data is the name used for data that Spotlight could not categorize. You won’t find a file or folder that is labeled System Data as it can be anything including your data.

System Data does include local Time Machine Snapshots. It can also be runaway cache or log files.

You’d have to ask Disk Inventory X developer what that blob represents.

You might try reindexing Spotlight as it might be miscategorizing some things.

Rebuild the Spotlight index on your Mac - Apple Support

ryzieisq wrote:

How can I clean at least half of it? I really need more space on my laptop

Apple’s final word on managing the “System Data” category:

System Data: Contains files not listed here. It includes system files like log files, caches, VM files, and runtime resources. Temporary files, fonts, app support files, and plug-ins are also included.

You can’t manage this category’s contents. macOS manages them, and the size varies based on your Mac’s state.

Storage issues on MacBook - Apple Community

ryzieisq wrote:

How can I clean at least half of it? I really need more space on my laptop

You would need to figure out what it is. It could entirely be your data that cannot be identified correctly. As I stated, you’ll have to ask the makers of Disk Directory X what that large container of other files means.

ryzieisq wrote:

How can I clean at least half of it? I really need more space on my laptop

You don't have control of system data. There are ways to get rid of it, but that's usually just temporary.

My Russian is a bit rusty, but it look like you only have 70 GB of "system data". That's very good. In theory, it might be possible to recover maybe 30 GB of that. But that really only means that you are totally out of disk space.

Save your money for a larger hard drive on your next computer. Until then, you can delete old files you don't need and/or archive them to an external hard drive.

You can use more iCloud features to offload some storage to the cloud. This will work, but it requires faith and an "all in" approach. With all iCloud space-saving features enabled, you will run with less free storage than you would like. You will have to be very aggressive about deleting files in non-iCloud locations.

ryzieisq wrote:

How can I clean at least half of it? I really need more space on my laptop

One of my Macs has 80 GB and the other has 90 GB in "System Data." I think this is normal, you have 70 GB, which is even better. Since you can't really reduce "System Data" yourself, you will have to consider moving other things (Photos, Music ...) off your main drive onto an external drive. Or you can use the options Apple offers for storing some things in iCloud. As an example, iCloud Photos allows you to have iCloud store all the full size images in your Photos Library and on your Mac only tiny thumb nails are physically stored. When my daughter did this it reduced her storage by 90 GB.

One thing you can do is empty the Trash. If you have lots in the Trash, emptying it can reduce "System Data" by that amount.