Execution Context:

Siebel Server Scripts are executed in Siebel Application Servers by Application Object Manager (AOM).
Siebel Browser Scripts are executed at Client Side by Client Browser (Internet Explorer)

Files:

Siebel Browser Scripts are converted in JS (Java Script) files and stored in PUBLIC\ENU directory.
No JS files are created for Server Scripts and they are executing using Siebel Scripting [...]

return(CancelOperation) statement is used to send the control back to Siebel or in other words to stop the normal flow of execution of script of a particular event.
This statement is mandatory if we are defining a custom method. For example:

There can be several variations to the above questions such as

How can one call a Workflow synchronously from a script without using Business Service: Workflow Process Manager?
What are various ways business services that we can use to call a workflow process in scripting?

The Answer to the above question is