Just wondering an Insight in the difference between these two indexes

IdKey / PrimaryKey
================= 

Property Identifier As %Integer

Index Index1 on Identifier [Idkey]

Index Index2 on Identifier [PrimaryKey]

What's the difference?

1. If I don't have Index1 and only have Index2,  then cache does still make its own id.
So how and why  do I ever use the PrimaryKey.  In Joins ??

Table1.Identifier = Table2.Identifier instead of Table1.Id = Table2.id ??
But I can still use Table1.Id = Table2.Id as cache still made one ID field

So where is PrimaryKey useful in cache?

2. If I declare Index1 , I am not able to have any Bitmap indexes [Cache throws an error on compilation saying I have an Idkey index]