> ## Documentation Index
> Fetch the complete documentation index at: https://docs.inboxapp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Concepts

> System lists, Contacted, and reachable vs unreachable leads.

<CardGroup cols={2}>
  <Card title="System lists" href="/help/leads-management/concepts/understanding-system-lists">
    Contacted and Mutual are automatic lists Inboxapp maintains from your messaging history and follows.
  </Card>

  <Card title="Reachable vs unreachable leads" href="/help/leads-management/concepts/understanding-reachable-vs-unreachable-leads">
    Why some leads can’t receive DMs, how Inboxapp filters them, and what to do when too many are removed.
  </Card>

  <Card title="How the Contacted list prevents duplicates" href="/help/leads-management/concepts/how-the-contacted-list-protects-you-from-duplicate-messaging">
    Contacted is a system list of everyone you’ve messaged; campaigns use it to avoid messaging the same person twice.
  </Card>
</CardGroup>