sc-status is a protocol status code from the appliance to a client.
The usual results for sc-status are the following HTTP response codes:

• 200
• 302
• 403
• 401
• 503

0 (zero) indicates that the ProxySG appliance did not have the chance to send the status code due to a possible connection reset. If you are logging s-action on the same log line, you might see TCP_NC_MISS_RST that could correlated to sc-status 0.
A packet capture taken when the sc-status=0 logging is happening can also help.