Cannot understand external drive configuration?

> and there is an "Other Volume" of 0.86TB

You're misreading what it's telling you:

You have a 0.86 GIGAbyte 'other' volume, not 0.86 TB - it's actually a 0.00086 TB volume. Hardly material.

As for what it is, it's likely an artifact of the formatting used on the drive.

For example, if the drive in question is part of a RAID array, the drives are formatted to the lowest common denominator (e.g. if you have an array with a number of drives that report 15.9TB usable space and one drive that reports at 15.8 (due to different model number, brand, etc.), then the extra 0.1TB on the larger drives would be stranded to provide uniformity over the array.

It could also be something to do with the way the drive was formatted - e.g. if there was an existing partition on the drive when you created a new one, vs. a complete wipe-and-reformat of the drive.

Without knowing more about the setup it's hard to be more specific, but either way you're not losing a lot of space,

If you are comfortable using the Terminal app, open it and run this command:

diskutil list

It will give more details about each of the drives; post a copy here. It's just technical detail, no personal info.

Did you get the Seagate Hub for Mac model? If so, according to its User Manual it comes pre-formatted in HFS+

Did you then reformat it for APFS? Since this Hub has a built-in HDD, HFS+ would still be the more efficient formatting. APFS was designed for SSDs.

Seagate, like a number of other drive manufacturers, tend to include "hidden" partitions that are typically small, and this could be what this "Other Volume" is.

In Finder what does a Get Info on the drive or folder say?