You use the df command when you want concise information about used and available space in volumes or aggregates. If you want a detailed breakdown of space usage by feature in a volume, aggregate, or a volume's footprint within an aggregate, you use the space usage commands.
The df command is useful if you want a quick view of how much space each volume has available or used.
You use the df command (or volume show and aggregate show commands) to see total space, available space, and used space. If you need more information about how or why space is being used in your volume or aggregate, you use the show-space and show-footprint commands (the space usage commands) for the volume or aggregate.
The space usage commands, on the other hand, provide much more detail about used space and what Data ONTAP capability is causing the space to be used. For example, they could be used to help you understand why the df command output shows used space even though there is no data in a volume.
Used space is dynamic, even for a system that is not being accessed by clients. For this reason, you should not try to compare the output of two different space commands, or even the same command invoked twice, too closely.