Difference between GUID and UniqueId

Question

What is the difference between GUID and UniqueId? They are both globally unique ids and from the client side we can see they are not the same:

ctx = new SP.ClientContext();
web = ctx.get_web();
list = web.get_lists().getByTitle("Issues");
item = list.getItemById(1);
ctx.load(item);
ctx.executeQueryAsync(function() { 
  console.log("GUID: " + item.get_item("GUID").toString());
  console.log("UniqueId: " + item.get_item("UniqueId").toString());
  console.log("ID: " + item.get_item("ID").toString());
});

Results in:

GUID: d2bcec49-5e88-494a-a141-4f4275363d79
UniqueId: 98237e32-67aa-4f95-887c-7567d0ab714e
ID: 1

Server side has SPList.GetItemByUniqueId so does that mean generally UniqueId should be used?

Answer 1

Take a look at http://msdn.microsoft.com/en-us/library/dd961684(v=office.12).aspx.

Guid
Specifies a user-customizable unique identifier of an item.

UniqueId
Document Lookup to GUID
Lookup to a GUID for an item.