Some of the subdirectories in the ACME code contain code that was brought in from another repository. List them all.

external any subdirectory in ACME whose entire content is from another repository
upstream master the repository everyone is sending their changes to. Only one so that everyone pulling from the same source is working with all the same changes. The source of an external.

Methods for including externals

submodule a link that points to another git repository. See Sharing External Code using Git Submodules . Submodules require an extra step after cloning to get the code.
subtree a set of files and commits from another git repo, brought in to your repo. See Sharing External Code using Git Subtree. Subtrees are included in ACME when you "git clone".
monorepo adding multiple projects, related or not, in the same repo. Code is included when you clone ACME.
buildtime the repo is checked out to the $CIME_OUTPUT_ROOT/$CASE/bld when compiling a case for the first time. Code does not exist in ACME repo.

Rules for adding externals

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Name	Subdirectory in ACME	external method	upstream master	development method	POC
CICE column physics	TBD	TBD	?	?	?
sBeTR for land model	components/clm/src/external_models/sbetr	submodule	https://github.com/ACME-Climate/sbetr	strict 1-way	Gautam Bisht
GCAM	TBD	submodule?	https://github.com/JGCRI/gcam-core	strict 1-way	Katherine Calvin (Unlicensed)

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