---
title: RAG L0 Domain Entity Hint
keywords: [L0 keyword matching, final retrieval result, domain detector, entity extractor, metadata filter]
summary: Define L0 as a query transformation hint layer instead of final retrieval evidence.
category: rag
source: rag-l0-domain-entity-hint
breadcrumb: RAG > L0 > Domain Entity Hint
kb_scope: rag-eval
covers: [l0 hint, query transformation, metadata filter]
when_to_retrieve: Use when a question asks whether L0 should decide final retrieval or only provide hints.
---

# RAG

## L0

### Domain Entity Hint

L0 keyword matching should not decide the final retrieval result.
In the modular RAG pipeline, L0 behaves like a lightweight domain detector and entity extractor.

The output can provide:

1. Candidate domain hints.
2. Matched entities and keywords.
3. An optional metadata filter for the first vector retrieval attempt.

Final evidence still comes from L1 vector retrieval, post-retrieval normalization, rerank, and context packing.
The important terms are domain detector, entity extractor, and metadata filter.