Why I have 12 GB of "cancellable" space even if I am not optimizing space with iCloud?
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.