"Call" is not a prefix, it execution command.

a procedure parameters can be supplied two different ways, when some of the parameters can be null/optional or have default values. When you are using

invoke(binding,action)

you need to provide input values for all parameters of the procedure, and parameter positioning must match with procedure definition. when using parameter named syntax like

invoke(binding=>'HTTP')

you can omit the parameters where you want to use the default values, or pass in NULL. Also positioning of parameters is not significant.