A pattern is a container for a bunch of parts. And parts are in turn a container for the points, paths, and snippets of (a part of) your pattern. Parts can be re-used and mixed and matched to create other patterns, a powerful concept to build a pattern library.
If you design a T-shirt pattern with a
sleeve, each of
those would be a part. If you then wanted to make a long-sleeved version of
your T-shirt pattern, you only need to design a new sleeve part. You can re-use
back parts of your short-sleeved T-shirt pattern, as they did
When developing a FreeSewing pattern, you will spend most of your time designing the individual parts.