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.
submodule a link that points to another git repository. See Sharing External Code using Git Submodules or submodules vs. subtrees
subtree a set of files and commits from another git repo, brought in to your repo. See Sharing External Code using Git Subtree
monorepo multiple projects, related or not, in the same repo.

ACME Rules for new externals

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Table of Externals currently in ACME

Subdirectory in ACME	external method	upstream master	development method	POC	Notes
cime	subtree	https://github.com/ESMCI/cime	2-way	James Foucar
cime/externals/MCT	subtree	https://github.com/MCSclimate/MCT	mostly 1-way	Robert Jacob	usually brought in with CIME
cime/externals/pio*	subtree	https://github.com/NCAR/ParallelIO	mostly 1-way	Jayesh Krishna	usually brought in with CIME
components/mpas-o/model	submodule	https://github.com/ACME-Climate/MPAS clone of https://github.com/MPAS-Dev/MPAS	strict 1-way	Mark Petersen
components/mpasli/model	submodule	https://github.com/ACME-Climate/MPAS clone of https://github.com/MPAS-Dev/MPAS	strict 1-way	Matt Hoffman
components/mpas-cice/model	submodule	https://github.com/ACME-Climate/MPAS clone of https://github.com/MPAS-Dev/MPAS: branch cice/develop	strict 1-way	Adrian Turner
components/clm/src/external_models/fates	submodule	https://github.com/ACME-Climate/fates clone of https://github.com/NGEET/fates	strict 1-way ?	Gautam Bisht
components/clm/src/external_models/mpp	submodule	https://github.com/ACME-Climate/mpp branch alm/develop	strict 1-way ?	Gautam Bisht
components/homme	monorepo	NA	NA	Mark Taylor	ACME CAM uses homme/src/share and homme/src/preqx