This method is used together with a variable of the WEBSERVICE data type to select a web service operation to be invoked:
<ws_var>.LoadMeta("service name", "port name", operation name")
You need to execute the loadMeta() method for this variable before you would be able to use this method. It accepts three arguments: the name of the web service currently in use, the port name (Lycia offers two ports SOAP 1.1 and SOAP 1.2, your webservice client can use both of them or any particular one), and the name of the web operation you want to execute:
CALL ws_var.SelectOperation("LengthUnit", "LengthUnitSoap", "ChangeLengthUnit")