The svn:externals property lets one working-copy tree include additional versioned directories or files from another repository location.
An external is effectively another checkout managed as part of the surrounding working-copy workflow.
Typical uses include:
The external definition is versioned because it is stored in a normal Subversion property.
An external that follows a moving HEAD can change even when the parent project’s own revision has not changed.
For reproducible historical builds, define externals so the dependency can be recreated at a known revision when practical.
This is especially important for release branches and tags.
When a directory with svn:externals is updated, Subversion can also update the external working copies described by that property.
Therefore an apparently simple parent update may contact additional repository URLs and change files beneath external directories.
Externals may refer to another location in the same repository or to a different repository.
Cross-repository externals have an important consequence: repository commits cannot become one atomic revision spanning two independent repositories. Changes in a different-repository external must be committed to that repository separately.
Changing svn:externals changes the definition, not necessarily every already-checked-out external immediately.
After committing or receiving a changed definition, update the parent working copy so Subversion can synchronize the external checkout.
External path already exists — an unversioned or unrelated local directory may obstruct the checkout target.
Authentication succeeds for the parent but fails for the external — the external may use a different server/repository and therefore different credentials.
Build changes unexpectedly after update — check whether an unpinned external advanced to a newer revision.
Commit does not include external changes — verify whether the external is a separate working copy and whether it points to a different repository.
External URL stopped working after repository move — inspect whether the definition uses an absolute URL or an appropriate relative URL.
A tag that depends on floating externals may not reproduce the dependency versions that existed when the tag was created. Review external definitions when creating long-lived release snapshots.