I am not using iCloud drive, and I am neither optimizing disk space with iCloud. But the disk utility app shows that I have 12.62 GB of "cancellable" disk space. I thought that this space was due to the fact that some files could be safely deleted from the disk because they're backed in iCloud. But why do I have "cancellable" storage if I am not using iCloud to optimize disk space?
This is a new feature in macOS Sierra called "purgeable space". Aside from files safely stored in iCloud (when the "Store in iCloud" option is enabled), purgeable space can also include cache and temporary files that are no longer required by the system.
It can also include other files that aren't entirely necessary, like iOS software updates, local Time Machine data, older email attachments, and others.