If you plan to run one version of a job ad on a job board, then it's best to simply create a tracking link. However, if you plan to run different versions of an ad for the same job on the same job board, use New Ad Campaign.
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Posting jobs ads to external job boards is a form of marketing. As that is the case, it's a good idea to try out different campaigns so that you can determine what approach attracts the most quality candidates. In Greenhouse, this can be done via Sourcing > Job Board > New Ad Campaign. Click here for step-by-step instructions.
For example, let's say you've created a series of job ads for your "Software Architect" job that highlight your companies work/life balance, and you've created another series of job ads for the same job that highlight the awesome people at your company. Creating campaigns allows you to compare the effectiveness of your job ads side-by-side via the Job Board page under the Sourcing tab:
... as well as via the "Candidates by Campaign" report on the Reports tab: