If you choose "Image", you can choose the option "Create New Volume": Yes or No

By default the option "No" is chosen

the new Virtual Machine will be created with System Volume (Root Disk) defined in the flavor.

If you want to choose different size of the System Volume (Root Disk) - check the article "VM creation - option Create New Volume - Yes"

In contrast to VM created with option "Create New Volume - Yes" https://creodias.eu/-/vm-creation-option-create-new-volume-yes, the system disk is "ephemeral" - it is not visible in Volumes pane