I am not sure if everyone here has seen this page yet as I only just found it myself. It explains a lot of things we have questioned on many occasions and would be worth the read for anyone making parts. It was written for manual parts creation like @vanepp does but a lot of what it talks about is also useful for those of us that use the parts editor.
https://github.com/fritzing/fritzing-app/wiki/2.1-Part-file-format Wiki page
If you do not want to read all of it or you always use the parts editor like I do I would still recommend you at least read the “about SVGs” section and the “more SVG topics” section.