When to use struct in C#?

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?

Source: Tips4all