While creating a vacancy, you can select from multiple different sources, including your Talent Pools, the Vault, and your Workforce.

Sources allow you to exert control over what candidates receive Recommendations - Be it from your Talent Pools, Vault or internal Workforce.

You can also set these up to be default under Settings, Sources. 

The total number of candidates filtered by the job description you added, can be seen within each source:

(You can also choose the Vault Recommendation mode for those accounts with a Vault Source but this will be only applicable for clients with a Bullhorn integration.)

Talent Pools

While in the process of creating your vacancy, you can select Talent Pools, that Candidates will have the opportunity to join.

Once a Candidate applies, they will be prompted to join the Talent Pool or Talent Pools you have selected

Other useful help docs: