Hi Roman,
it is actually both. Within the D&B mashup (which you need to create) you can search for accounts and add them to C4C:
When doing this additional D&B related data is replicated to C4C like sales volume and employee count which is persisted on the account master. You can also match existing C4C accounts with D&B via the unique DUNS number.
Details can be found under https://docs.dnb.com/static/doc-uploads/catalog/en-US/360/3.0/dnb360-setup-sapc4c.pdf.
Regards, Marcus