Looks like it's a combination of SSH server IP address + public key.
Each VM you create (up to 25 of them) gets a different CNAME record of the form s0NN.exe.xyz where NN ranges from 01 to 25. Each of these names, from s001.exe.xyz to s025.exe.xyz, resolves to a different IP address.
Therefore the individual VM can be distinguished this way, and the account they are associated with can be identified using the SSH public key that is used to authenticate.
Each VM you create (up to 25 of them) gets a different CNAME record of the form s0NN.exe.xyz where NN ranges from 01 to 25. Each of these names, from s001.exe.xyz to s025.exe.xyz, resolves to a different IP address.
Therefore the individual VM can be distinguished this way, and the account they are associated with can be identified using the SSH public key that is used to authenticate.