I am new to the use of Tixati and I have doubts about some concepts. In the general settings there is an option under Location to define "incomplete piece storage location". Is it possible to define a temporary folder per disk or category? I ask this because I have several disks where I download large files and if I concentrate all the temporary files on a single disk, in addition to the space occupied, when they are moved to the final folder, this process takes quite a long time if it is not on the same disk.

In the Categories options there are, in Local Files tab: Downloading and Seeding Location. What is the difference between these folders? In BitTorrent what you download in a folder is the same as what you share, so to my understanding both folders must always be the same.

In this case, your guesses are not quite right.

Incomplete pieces setting is not for data that is currently being downloaded (it would be extremely slow to save it to a temporary file before saving to proper location, and in-memory disk cache exists to deal with that anyway). It's a location for pieces that belong to multiple files, some marked for download by the user, and some not. As torrent clients (using version 1 protocol) can only share and validate full pieces, downloading some file (assuming that you also want it to be seeded to others in full, and that its size isn't an exact multiple of piece size) makes them download all of its pieces, and the first one and the last one most likely have extra data that belongs to other files, some of which might not be needed. However, the data is needed, and has to be stored somewhere.

Clients have different ways of doing it. Some simply create those extra files to save their start or end. On filesystems without sparse file support it can result in enormous waste of space (many gigabytes for less than a single piece), or they can suddenly inflate in physical size later if copying program or synchronization tool doesn't handle sparse files properly. Other programs might misbehave when they see an almost empty file without any expected format. Some torrent clients save such files with specific extensions (“.incomplete”) or put those files into specific hidden directory (“.unwanted”). Extra files still might cause inconveniences with other software, and having 10 proper files together with 15 not-so-proper ones might complicate, say, batch processing. Some clients have a dedicated location for that extra data, and you only get files you want at download location. The downside is that if you add many selectively downloaded big torrents, you might need some free space for that shared data catalog to grow.

More correct caption for the option would be “Partially needed pieces location”. Moving files after completion, as you've described, is handled by the option above it. If you turn categories on, you can have multiple such targets, and other automation. This is what “Seeding location” (i. e. place for completed torrents) does.

As a side note, I've also noticed some time ago that in some cases Tixati doesn't “garbage collect” those pieces if you download a subset of big number of files, then remove some, then download some more, remove some, etc. It keeps displaying presence of data at starts and ends of files that are no longer neighbors of any wanted file, and all set to “off”. Because they took some space, I had to stop the program, remove the piece files with a specific hash, then re-check the torrent and re-download some pieces. Not sure if it's still the same in the latest version, though.