Message Start is always a catch shape. That is, in order to use a Message Start, there has to be a corresponding Message Event that throws the message.
The shape in the Process that throws the message should be configured to identify the target Process and the target Element.
The Intermediate Event that throws the message and the Message Start that catches the message should have the exact same name.
Signal Start is always a catch shape. That is, in order to use a Signal Start, there has to be a Signal Event that throws the message.
A signal is sent to any Process that can receive it, but it does not have a specific Source and Target. Any catch shape with the same name will be triggered.
The throw Signal Intermediate Event that broadcasts the signal and the Signal Start that catches it should have the exact same name.