crypto::system it describes a Windows System Certificate. It as the following command line arguments:
/file:the path of the certificate/export:export to a .der file
The following output was taken from this guide:
mimikatz # crypto::system /file:"SystemCertificates\My\Certificates\096BA4D021B50F5E78F2B9854A7461678EDAA006" /export
...
Key Container : d209e940-6952-4c9d-b906-372d5a3dbd50
Provider : Microsoft Enhanced Cryptographic Provider v1.0
...
Saved to file: 096BA4D021B50F5E78F2B9854A7461678EDAA006.der