This parameter is used to validate callbacks and webhooks coming from Forte, And it responds on every request.
The information this parameter will contain depends on integration method you are using.
The example below displays what information is included when using Forte Checkout:
HMACMD5("api_login_id|method|version_number|total_amount|utc_time|order_number|
customer_token|paymethod_token|trace_number","Secure Transaction Key")
This parameter must be verified to confirm the validity of the returned message (for both callbacks and webhooks). An invalidated response signature could present an exploitable weakness in your code.