Difference between Branch merging and Stream_to_stream merging in perforce

Question

When I am trying to merge between two streams in Perforce, for example Stream A is a parent and Stream B is a child of Stream A. I want to merge down to Stream B from Stream A. When I try to merge between these, in the merge dialog box there are three options

1. stream to stream merge
2. specify source and target files
3. use branch mapping

I have a question here, what is the difference between these 1st and 3rd?

Answer 1

These are all just different ways to specify a source and target mapping.

1. here are my streams, figure out how they relate and merge one into the other
2. here are two sets of files, merge one into the other
3. here is a branch mapping defining two sets of files, merge one into the other

The purpose of having streams is largely to make #2 and #3 obsolete -- when you merge two streams a branch mapping is automatically generated internally to perform the merge. If you don't use streams you define your own branch mappings (which gives you some flexibility but is also more work).