What can ZFS reservation be used for?

Question

ZFS

Hello. There was a question on work with ZFS. The main task is to prevent the data on the network storage under nas4free from filling up the entire pool on ZFS, as this leads to speed degradation. I have already set quotas, but not all software works correctly with a network drive, since it seems that 10 terabytes are available, but in fact the quota is 9. And the program that writes over the network does not rotate its files.
I also dug up a ZFS reservation, but I didn’t really understand what it was for and whether it would suit my task - to always leave an area of ​​​​1 terabyte free. I read the ZFS administration manual, it didn’t become clearer :(.

Setting up a ZFS reserved space involves allocating
pool space that is guaranteed to be available to a dataset.

and which dataset has no explanation.