Subprojects are ordinary Planfix projects with the system field Parent project filled in:
There's no other difference between a subproject and a regular project.
When coupled with Project groups, the purpose of subprojects is to improve the visual understanding of a company's project structure. In a list of projects, subprojects can be displayed as a tree, which helps you visually display how projects are nested under one another:
In theory, you could have as many subprojects in Planfix as you wanted, but in practice companies usually don't have more than 2-3 levels of nesting.