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Feature is always in the ACME source but may not be fully working.  Once working and "officially added", test for that feature becomes part of standard test suite Feature developer is always working with recent master to avoid divergence.   Least painful integration.   Developers of other features can build off of what you've committed to master.

Long-lived integration branch (for 2 or more related features)

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Feature lives on the integration branch until integration branch is merged to upstream master.  Maintaining the integration branch (with merges from upstream master) could become arbitrarily hard if related codes diverge.  Allows team of developers to work on a large speculative feature.

Long-lived branch off of ACME-Climate/ACME master (or from an integration branch)

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Features lives on your branch until it is finished.   Maintaining the branch (with rebases or occasional merge from master) could become arbitrarily hard if related codes diverge.  Other developers unaware of feature until its finished.