Sources are the organization-defined dictionary of channels through which candidates enter the pipeline — job boards (e.g. LinkedIn, Indeed), agencies, referrals, internal applicants, prospecting tools, events, and other ways an application is attributed. Each application references a single source via source_id; resolve the human-readable label and its broader category (sourcing strategy) here. The response is a mix of Greenhouse's built-in global sources and any custom sources the organization has configured under Configure > Custom Options > Sources, so the available set varies between accounts. Distinct from tracking_links, which are URLs that auto-tag a source on incoming applications.