flagd supports selectors, so separate flags into different buckets, eg. each file source, http source, etc, is its own selector.
If flagd is started with multiple sources, the selector param can be used to evaluate/sync only the set of flags from the source in question:

{
    "flags": {
        "flagb": {
            "state": "ENABLED",
            "variants": {
                "foo": "foo",
                "bar": "bar"
            },
            "defaultVariant": "foo"
        }
    },
    "metadata": {
        "flagSetId": "b"
    }
}

{
    "flags": {
        "flaga": {
            "state": "ENABLED",
            "variants": {
                "foo": "foo",
                "bar": "bar"
            },
            "defaultVariant": "foo"
        }
    },
    "metadata": {
        "flagSetId": "a"
    }
}

docker run \
  --rm -it \
  --name flagd \
  -p 8015:8015 \
  -v $(pwd):/flags \
  ghcr.io/open-feature/flagd:latest start \
  --uri file:./flags/a.json \
  --uri file:./flags/b.json

curl -X POST 'http://localhost:8016/ofrep/v1/evaluate/flags/flaga'

curl -X POST 'http://localhost:8016/ofrep/v1/evaluate/flags/flagb'

curl -X POST 'http://localhost:8016/ofrep/v1/evaluate/flags'

We want to support a "selector" field similar to what's in the sync protocol, as a header, which would restrict evaluations to only the flag set identified by the header. ie:

curl -H 'X-Flag-Set-Id:a' -X POST 'http://localhost:8016/ofrep/v1/evaluate/flagb' // not found since flagb is not in set a.

Note, this requires #1610 to be done first.

The precise name of the header is subject to discussion, but feel free to make a proposal or use what I've suggested above.