The Signals form is where the default connection properties for signals are set.

See also Line Styles Form and the Arrows Form

When a connection is first created, it's appearance is that defined for the Signal type for the new connection.

The Signal for a new connection is dependent on how that connection was created.

If it was made by Linking connection points then the signal is the "Iam" signal for the output at the origin of the connection.

If the connection was made by  Dragging from one symbol to another then the signal is the default signal for the class of the symbol at the origin of the connection, as defined in the Class Table.