What does it mean 'upstream' in the manifest file, repo?

Question

I found the 'upstream' attribute in manifest file, repo. I'm not sure what it means during 'repo sync' and 'repo sync -c' And what's the difference between projectA and projectB below example.

During 'repo sync', I think it will clone projectA with commit '12345', In projectB case, it will also clone projectB with commit 'abcde'. Why 'upstream' is needed?

Please could you explain what it means?

<?xml version="1.0" encoding="UTF-8"?>
<manifest>
    <project name="projectA" path="project/A" remote="aosp" revision="12345" upstream="projectA/develop"/>
    <project name="projectB" path="project/B" remote="aosp" revision="abcde" />
</manifest>

Answer 1

Attribute upstream as defined in repo Manifest Format

Attribute upstream: Name of the Git ref in which a sha1 can be found. Used when syncing a revision locked manifest in -c mode to avoid having to sync the entire ref space.

From this I understand that the sha mentioned in revision="12345" can be found in branch develop

I am guessing that there is some default entry at the top of manifest which will help the second line in your snippet.