According to Apple, optimized Storage in macOS Sierra and later can save space by storing your content in iCloud and making it available on demand. When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in iCloud automatically. Each file stays right where you last saved it, and downloads when you open it. Files that you’ve used recently remain on your Mac, along with optimized versions of your photos.
You can transfer older files to iCloud by manually dragging files from your local Documents folder to a folder you name ArchiveDocuments if you wish. For example, you can free up local space by dragging all of your file from 2016 to folder you could name "Documents2016."
Lets say you move 3 GB worth of files from your local Mac to iCloud. You might not see the 3 GB extra space immediately. It will likely show up after a restart, but you can also prompt the extra space to display by opening the Storage Management window:
Access the the Storage Management menu from Apple Menu: About This Mac: Storage Tab (in the Storage Management). On slower Macs it may take awhile for space capacity, partitions and availability to display. You will see "calculating" perhaps for a few minutes before the information properly displays. When it displays, you will see the latest space available with the extra 3 GB available.
Store in iCloud
Click the Store in iCloud button, then choose from these options:
Desktop and Documents. Store all files from these two locations in iCloud Drive. When storage space is needed, only the files you recently opened are kept on your Mac, so that you can easily work offline. Files stored only in iCloud show a download icon , which you can double-click to download the original file. Learn more about this feature.
Photos. Store all original, full-resolution photos and videos in iCloud Photos. When storage space is needed, only space-saving (optimized) versions of photos are kept on your Mac. To download the original photo or video, just open it.
Messages. Store all messages and attachments in iCloud. When storage space is needed, only the messages and attachments you recently opened are kept on your Mac. Learn more about Messages in iCloud.
Storing files in iCloud uses the storage space in your iCloud storage plan. If you reach or exceed your iCloud storage limit, you can either buy more iCloud storage or make more iCloud storage available. iCloud storage starts at 50GB for $0.99 (USD) a month, and you can purchase additional storage directly from your Apple device. Learn more about prices in your region. In March 2020 2 TB of storage costs $9.99 per month (50GB: $0.99 and 200GB: $2.99).
Optimize Storage
Click the Optimize button, then choose from these options.
Automatically remove watched movies and TV shows. When storage space is needed, movies or TV shows that you purchased from the iTunes Store and already watched are removed from your Mac. Click the download icon next to a movie or TV show to download it again.
Download only recent attachments. Mail automatically downloads only the attachments that you recently received. You can manually download any attachments at any time by opening the email or attachment, or saving the attachment to your Mac.
Don't automatically download attachments. Mail downloads an attachment only when you open the email or attachment, or save the attachment to your Mac.
Optimizing storage for movies, TV shows, and email attachments doesn't require iCloud storage space.
See more at support.apple ...
How to free up storage space on your Mac ...
