Hi, M².
What Ean86 says is correct. But you have to keep in mind that he is referring to two different syncing processes:
1. Bluetooth connection that directly involves the Perdix and but no online server (Cloud)
2. Internet connection that does not involve the Perdix directly but connects to an online server (Cloud)
We keep these two connection processes separate in order to respect privacy concerns of users that do not wish to have their personal dive information transmitted over the internet nor stored in an online server. For the sake of simplificity lets call connection #1 Bluetooth download or just download and we can call connection #2 Internet Sync or just sync.
The first step is to do a Bluetooth download. Keep in mind that successful completion of this connection only puts dive information locally into the one device doing the downloading (ie mobile device, desktop or laptop). You can enter details about the dive into said device. Again those details will only be local to this one device if you do not do connection #2 and perform an internet sync. Some users prefer to keep it this way and do not want their information in cyberspace. If you then go and do another Bluetooth download into a second device, this 2nd device will not have any of the dive details you entered into the first one because you never did a #2 Internet Sync. The cloud was never engaged.
In order to engage the cloud and do an Internet sync, you have to create a Shearwater Cloud Account. After doing a Bluetooth download and manually adding your dive details you have to complete a #2 Internet sync. On mobile devices you have to go to the mask and regulator icon called "Profile", login to your account, and then tap on the blue cloud symbol with two circular arrows inside it.
After doing this successfully, you can go into any other device(s), complete a #2 Internet sync and the dive details you manually entered into your first device will now be available. If you want to add or correct further details on this second device, you will need to do another #2 Internet sync to have those new details up in the cloud and accessible to other devices.
I hope this helps.