Hello.
tell application "Microsoft Excel"
Where can I find complete help for all Excel commands?
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tell application "Microsoft Excel" - full syntax.
I suspect you're using Apple's Script Editor app.
In Script Editor, to open an application's dictionary, go to
File > Open Dictionary…
Search for "Microsoft Excel.app", same for Word and the others.
Otherwise, there is no official Microsoft page for that. Google is your best friend to find examples.