Note: depending on where you are, the concept of money being withheld on invoices to claim from your customer may be called either retention or holdback. In this article, we'll refer to it as retention.
Retentions are set up at the point of converting an Estimate to a Job. This will be at the bottom of the pop up that appears once you accept the Estimate.
Retention is part of the "Completion percent" invoicing. Note you can opt to have retention calculated with or without the variations on the job, please head here for more info on this.