Under the covers BEF uses the separately managed Bazel Java SDK for all interactions with Bazel. Primary development for the SDK should occur in that repository.
However, the IDE work happening in this repository is largely engaged with making improvements and fixes in the SDK. Roughly 90% of the code to support Bazel in Eclipse is not specific to Eclipse, and therefore lives in the SDK.
For that reason, a vendored (static copy) version of the SDK lives in this repository. As changes are made to the SDK in either repository, the changes are copied across using scripts such as sdk_copybara.sh.
More details can be found in these places: