Client-side versus server-side APIs
Originally, ServiceNow's name was GlideSoft. While that was a long time ago, there are still some indicators of the company's history, such as in the naming of ServiceNow's scripting API suite: the Glide API.
Scripting is an important part of ServiceNow, and many different types of records support scripting in one or more fields. Some scripts execute server side, and some execute client side. Whether a script executes on the client or on the server, determines the API that it has access to.
Server scripts, executing on the server as they do, have access to a different set of programming interfaces than client scripts, because client scripts execute within the browser. Thus, only scripts which are included in the webpage and sent from the server to the browser can be executed or called from within client scripts. Since it would be impractical and have a negative impact on performance to send over the entire scripting library from the server to the client...
