When you make a change to the file on one computer iCloud will detect that change. It then copies the file up to the cloud. On the second computer iCloud detects that the cloud version is newer and will copy the file down to the second computer.
The problem is if you make changes on the second computer before iCloud can complete the copy. QuickBooks Mac 2019 will detect that and give you the standard iCloud "changed copy" dialog. You get to choose which version to accept: the one coming from the cloud or the one you are modifying locally.
In practice I have my Quickbooks company file open on both my laptop and my desktop. The laptop is closed. I'm working on the desktop, adding transactions, etc. Then I head out to a coffee shop with the laptop.
When I open the laptop up, as long as iCloud has had time to sync, my open copy of QuickBooks Mac will have all the changes I made on the desktop.
If I start entering something on the laptop before the sync has occurred I'll get the conflict dialog and be asked to choose either the copy from the desktop or the copy from the laptop.
In practice I find it works really well - generally the sync has occurred before I modify anything and I can just pick up where I left off on the other computer.
But if you shut down the desktop before iCloud can finish uploading or if you had the laptop unplugged and sleeping (and thus didn't have background updating while sleeping running) you might have more latency in the update. The same thing can happen if the two computers have masses changes in other files (like a big movie has been added to one that is shared via iCloud). This kind of thing can slow down the iCloud copying process and cause more latency.
