The Zoho Service Communication Key or the ZSC Key is a key to communicate with other Zoho services. Invoices and Estimates created in Zoho Invoice can be viewed in other Zoho Services using the ZSC Key. For example, if you are a Zoho CRM user from within the CRM account, you can view all the invoices and estimates created for a contact using the ZSC Key.
How do I generate a ZSC Key?
Do refer to the below image for further understanding.
Does each organization has a unique ZSC Key?
The ZSC key for all your organizations will be the same. The ZSC key is specific to a user. There is a unique ZSC key for each admin user in your Zoho Invoice account.
Zoho Invoice FAQ