>>No, COM servers can have UI. VFP DLL COM Servers don't support UI, but an EXE can have UI. You can call Excel, Word, Visio, and others as COM servers and all can have a UI.
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>Ok, my bad, I was thinking business objects when I said COM server.
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>Would we say "Components shouldn't cross tier boundries.", in a multi-tier application?
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>In other words a UI tier customer class (data entry form) would not include the business tier customer class. It would just be able to commumicate with that component.

Tier boundaries are sometimes fuzzy. Lets say you have an edit mask on a field that formats a phone number. You could have a business rule that says "Anytime you have a phone number, it is formatted as ###-###-####." To further complicate things, the mask can be stored in the data dictionary (DBC for VFP data) and you can pull this out at runtime. Which tier does this belong in? You could argue for any of the three. My personal feeling is that it belongs in the UI since it only affects how to see the data.