If you've given a real email address to EaseUS in the past you might get an email titled: "How to extend C drive or system partition in Windows 10?" I'm a little concerned it'll lead some people into trouble.
When it comes to disk use, Windows 10 is a bit of an odd beast. Upgrading builds, which are essentially new versions of Windows, can create extra partitions. Installing 10 can add partitions in different orders, and later builds may even complain about that order. Windows 10 also has newer disk storage features that are less documented & not as well or as commonly understood. And when it comes to storage location, Windows 10 Store apps may not behave according to how things are supposed to work in 10.
That stuff, combined with 10's new GUI & preferred ways of doing things can be confusing enough, and this email with too few details might lead some folks to do something they shouldn't.
The email from EaseUS talks about increasing the size of the Windows or system partition. If you add the capacity of each partition up it should equal the total disk capacity. To make one partition larger another partition(s) has to become smaller. Regardless the Windows version, making the Windows partition larger means shrinking some other partition on the hard drive -- if there are no other partitions, or the only partitions there are the default 4 that 10 adds during a fresh install, what you've got is what you've got, & it's all you'll have unless you add more storage.
The disk storage in any device is used to its max capacity from the manufacturer -- if there's an unformatted portion of the hard drive storage it's there for a reason [likely as an area to swap out defective areas of the main storage]. If a hard drive has been added or replaced, or if partitions have been added, whomever did that could leave some unformatted, unallocated disk space, But Why Would They?
If you're running out of space on the partition where Windows lives, basically you can remove stuff, &/or add file compression, &/or add more storage capacity. You can find articles & tutorials & even some tools to help make Windows itself smaller, e.g. removing features you do not use. You can use 10's compact.exe, or the disk compression checkbox in the properties dialog for most any folder in Windows 7 +. You can sometimes increase internal storage, adding or upgrading hard drives.
Adding external drives works better for some stuff than others. If you put your music or video or docs on an external drive that's normally no problem, though it might take longer to write the files since USB is slower than an internal SATA III drive. [eSATA is as fast, but becoming rarer every day, while newer tech *should* be faster, *when* you can buy reasonably priced hardware.] The problem adding software to external drives is that the drive letter varies -- many apps for example will not install to a USB drive because of that, though you can add & mount a VHD on external storage & Windows will see it as an ordinary drive.
RE: Windows 10... Windows 10 likes a separate partition to store or hold the boot files necessary to get things started when you turn on your Windows device. In a multiboot system with more than one copy of Windows that can be the 1st partition, but normally only if the disk & partitions are MBR rather than GPT, using a legacy bios or legacy bios *mode*. On a GPT disk using UEFI bios the boot partition will be hidden & formatted FAT32. There *may* be a hidden Microsoft Reserved partition that will show up in software as an unknown format, & there usually is a hidden Recovery partition with minimal setup files. Then there's the regular, Windows or system partition, and it's possible for the manufacturer to add one or more partitions, often hidden, that store versions of the software they add to new systems or devices.