JPG is what almost every photo arrives as, so this is the format most splits start and finish in.
The thing to understand about splitting a JPG is that the tiles are new files. A JPG cannot be cut without being decoded to pixels and encoded again, so each tile goes through exactly one re-encode. At the default quality of 92 that is very hard to see on photographic content, and one generation is the least it can possibly be. What causes visible damage is doing it repeatedly: splitting a tile, editing it, saving it, splitting again. Each pass costs a little.
The quality slider is there for the cases where the default is not right. Push it up toward 100 for a tile that is going to be edited further, or down if the tiles are going somewhere that will recompress them anyway.
One thing that does not survive: metadata. The tiles are drawn on a canvas and written fresh, so EXIF from the original, camera model, exposure, GPS, is not carried into them. That is a privacy advantage more often than a loss, but if you need the metadata, keep the source file.
Rotation is handled properly. A photo shot sideways carries an orientation flag, and it is applied when the file is decoded, so the tiles come out the right way up.
Nothing is uploaded.