What's the difference? If you lose a python developer, for example, you've lost the expertise of python development (which you can recover by hiring a new developer). But, if that developer knew the quirks of the accounting software that your company sells, and can quickly resolve bugs, then that is institutional knowledge that will take time and experience for someone to learn.
To guard against this, you need to encourage employees to document their knowledge of your applications and work processes. The worst way to do this is through requiring them to create complex, formal documents. Your workers are already busy, and this will get put off.
The best way is to encourage them to informally record what they learned as they solve issues. Blogs and wikis are ideal formats.
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