Intellij maven options - What are differences between them

Im a bit confused with maven options in intellij.

What are the differences between and when would you use them?

• Reimport
• Generate Sources and Update Folders
• Download Sources
• Download Documentation

• Reimport forces a reload of the POM. This should only be necessary if you have any major changes.
• Generate Sources generates code. Think WSDL to Java, etc.
• Downoad ... downloads the source or documentation packages for dependencies. Generally not required, but it can help if you try to debug your code and you want to jump into (nicely formatted and documented) dependencies or with writing proper JavaDocs.

PS: In rare cases when your project is really messed up and Reimport does not help, use File -> Invalidate Caches / Restart...