Notes on source with vectors

website/docs/usage/v3-1.md

@@ -220,4 +220,34 @@ working as expected, you can update the spaCy version requirements in the
 + "spacy_version": ">=3.0.0,<3.2.0",
 ```
 
-<!-- TODO: vectors initialization and anything else we want to mention -->
+### Sourcing pipeline components with vectors {#source-vectors}
+
+If you're sourcing a pipeline component that requires static vectors (for
+example, a tagger or parser from an `md` or `lg` pretrained pipeline), be sure
+to include the source model's vectors in the setting `[initialize.vectors]`. In
+spaCy v3.0, a bug allowed vectors to be loaded implicitly through `source`,
+however in v3.1 this setting must be provided explicitly as
+`[initialize.vectors]`:
+
+```ini
+### config.cfg (excerpt)
+[components.ner]
+source = "en_core_web_md"
+
+[initialize]
+vectors = "en_core_web_md"
+```
+
+<Infobox title="Important note" variant="warning">
+
+Each pipeline can only store one set of static vectors, so it's not possible to
+assemble a pipeline with components that were trained on different static
+vectors.
+
+</Infobox>
+
+[`spacy train`](/api/cli#train) and [`spacy assemble`](/api/cli#assemble) will
+provide warnings if the source and target pipelines don't contain the same
+vectors. If you are sourcing a rule-based component like an entity ruler or
+lemmatizer that does not use the vectors as a model feature, then this warning
+can be safely ignored.