Hello Sims
The “single-connection” keyword in the configuration of the TACACS server host command is used to maintain a single open TCP connection between the router and the server, rather than have the router open and close a TCP connection each time it must communicate. This improves the efficiency in the communication between the router and the server. more info on this can be found at the following Cisco documentation:
As for the default attribute syntax, you can see several examples in this TACACs documentation. Take a look at the section titled CONFIGURING DEFAULT AUTHORIZATION.
I hope this has been helpful!
Laz