What is difference between 'reference', 'safeReference' and ... ?

[thanksyouall]

I don't understand why different types of references are needed. When and what type should I use?

1. types.reference(Model):
I will get a "Failed to resolve reference" error if the object has not yet been created.
2. types.safeReference(Model):
I won't get the error, it's an option I like.

But then why do we need these ways:
3. types.reference(types.late(() => Model))
4. types.late(() => types.reference(Model))

[thanksyouall]

I understood. Late () is needed for circular references.
safeReference is needed for links that may not be loaded.