TC is a Word property that indicates if a DNS response message packet has been truncated when the length of the message packet exceeds the maximum permitted on the transmission channel. TC will contain the value 1 when the mesaage packet has been truncated, or the value 0 when the packet contains the entire response issued by the name server. 

Messages handled by the UDP-based DNS Resolver are restricted to 512 bytes (not counting the UDP headers). Longer messages are truncated and TC is set to 1 in the header. For this reason, the UDP-based TIdDNSResolver does not implement DNS zone transfers, but is the recommended method for standard queries in the Internet. 

TC is represented in a DNS header as a bit field. Changing the value of TC ensures that the appropriate bit field is updated in the DNS header for the message packet.