c# - When to use struct?

When should you use struct and not class in C#? My conceptual model is that structs are used in times when the item is merely a collection of value types. A way to logically hold them all together into a cohesive whole.

I came across these rules here:

• A struct should represent a single
  value.



• A struct should have a memory
  footprint less than 16 bytes.


• A struct should not be changed after
  creation.

Do these rules work? What does a struct mean semantically?