<Section title="Source in this channel's material">In the context of NotebookLM, a source is a document you upload into a notebook — journals, work documents, research quotes, transcripts — that becomes the material the model is an expert in [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@612]. Sources are the unit of grounding: everything the tool says is anchored to them, and it declines to answer questions that fall outside them [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@1458].</Section> <Section title="How sources behave in practice">- Each notebook supports up to 50 sources, each up to 500,000 words [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@763]. - Checkbox selection lets you control which sources the model listens to — the chat box displays counts like “15 sources” or “14 sources” so you can narrow focus to a single document [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@946]. - Every source gets an auto-generated source guide summary — very useful for a single-topic document, less so for a grab-bag of 800 quotes [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@884]. - Inline citations show which source passage produced each answer, and clicking one jumps straight to the original quote [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@1032]. - Source content is loaded into the model's context window — never used for training — which is why copyrighted material can be worked with safely [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@841].</Section> <Section title="Sources vs. notes">In the live Apollo 1 exploration, sources and notes act as separate contexts: you can focus the model on one or the other, but not both at once. The “suggest related ideas” prompt is the bridge — with a note selected, it finds related material inside the sources [^0cc0dd07-dc88-5d83-8cf7-2f6e48821796@2779].</Section> <KeyPoints>- A source is a user-supplied document that grounds a NotebookLM notebook; the model sticks to the facts those sources contain. - Sources are selectable (checkboxes), summarized (source guides), and cited (inline citations linking back to the exact passage). - Sources and notes are distinct contexts; use “suggest related ideas” to connect curated notes back to source material.</KeyPoints>