MS Excel as a TestComplete application? Can Excel be used as an application in TestComplete?

TestComplete can TECHNICALLY work with any application.  So, yes, you can "Test" Excel via TestComplete.  When TestComplete prompts for the "type" of application, simply select Generic Windows application.  

And yes, there are script extensions available for comparing Excel sheets.

Here's one link:

https://support.smartbear.com/viewarticle/42199/

And here's another:

https://support.smartbear.com/viewarticle/9046/

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 As a TestComplete rookie I'm unsure if MS Excel can be added as an application in TestComplete (and, if possible, as what kind of application?) and subsequently launched from the TestComplete interface in order to record test cases in MS Excel?

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Did you try it?  That's the quickest way to find things out about TestComplete.  If you don't get the results you expect, let us know what happened and we can help from there.