From the above link:
Retention refers to keeping or holding onto classified documents or material beyond the authorized period or without proper clearance.
Anecdote: Back when I started to work on black projects, a mentor told me the following story. He was working on a project report and turned it in to his boss Friday afternoon. Over the weekend, a few new ideas popped into his head. On Monday, he jotted them down and went to his boss. "Could I get my report back and attach these notes to it?" His boss replied, "Sorry, You're not cleared to see that."
It may have been an apocryphal story. But it carried an important message: What you may see, where you may see it and when are all important parts of a clearance.