By convention, requests which begin with a capital letter do not elicit a response from the server, while all others do. Unrecognized requests will always elicit a response from the server, even if that request begins with a capital letter.
The term command means a request which expects a response (except valid-requests). The general model is that the client transmits a great number of requests, but nothing happens until the very end when the client transmits a command. Although the intention is that transmitting several commands in one connection should be legal, existing servers probably have some bugs with some combinations of more than one command, and so clients may find it necessary to make several connections in some cases. This should be thought of as a workaround rather than a desired attribute of the protocol.