I can see some misunderstanding here so let me explain. As a server owner (VPS) you are responsible for maintenance of a server including hard disks, free space, OS, etc. Plesk is responsible solely for web sites. Though Plesk can assist with some of server routines in scope of web hosting, complete server management is not what Plesk is designed for.

That said, your 10Gb VPS includes a number of software entities pre-installed, including your OS, which all consume disk space, but are not a part of Plesk and are not controlled by Plesk (thus you shall not fully rely on Plesk for your server management). Normally disk space limit for VPS is not about "your own space" but about complete VPS size, including all those pre-installed software entities. You can double-check with your provider whether they really meant that you will get 10Gb of __free__ disk space in your VPS.

If you are looking for getting 10Gb of free disk space, than you may need to purchase shared hosting account ("customer" account) instead of VPS. In many cases it will cover your own files only, though yout provider has some flexibility on what shall be included in the limit. Also in this case Plesk will provide you report on what your disk space is used for (since it is all for web site now and all controlled by Plesk).

Hope it helps.

How can customers’ need to know what space is taken up by what be irrelevant?

It’s certainly very relevant if the plan partitioning is done by the software in question and that software has a dependancy overhead and together it consumes 1/2 of the space.

It’s certainly very relevant when no backups are taking that space ( an assumption on your part that they are ) and domain aliases are misreporting the same space being used instead of none, that calculations indicate data usage is doubled (and more) if there’s a domain alias, because that’s what it looks like to someone with their eyes open.

No, it’s not OK to pass the buck to the provider when inconvenient truths arise. It’s not OK to report 6.5Gb space being used when the only data I’ve added is 1.5Gb to a 10Gb plan. It’s your software, you tell me how much space it’s consuming, how much space the OS it depends upon is consuming, how much data I have, split the answer into the simples of OS, Application & Data and give user what they need, not what it’s convenient to deliver.

Then I can go to the provider and say something like this, “I rely upon Plesk and VPP for all operations and expect them to accurately inform me and what they are accurately informing me is that the plan you sold me doesn’t come with 10Gb of space for my own use, here’s the evidence; instead of what I have now, which is an irrelevant answer to the fact that it doesn’t do that job at all well.

It's OK if you have about 5Gb of disk space used by OS and soft. You can ask your provider to help you with understanding what exactly consumes disk space in your case.