A TestCredentials transaction validates the credentials passed in the transaction, but does not perform an action. TestCredentials should only be used at the beginning of the certification period to validate credentials and connectivity to the certification environment.
This should not be used as a "heartbeat" check and it is not required for running transactions.
The TestCredentials transaction includes the transaction request and response headers with only the transaction type in the Transaction block of the request and response. This represents the least of the possible Portico request and response messages.