Copying fields sometimes doesn't copy their content. But when?

[David Epstein]

Many thanks to Jacqueline for providing the master key to this  
question.  By trial and error, I think I have confirmed this rule:

"Copy <field or grouped fields> to <card, stack, or group>" will  
include the text of the field(s) in the new copy ONLY if
(a) the sharedText property of the field is TRUE, or
(b) the field is on the card that is or includes the destination, or
(c) the field is on, and destination is or is on, the first card of a  
stack.

Note that if you copy a field via the clipboard, the paste command  
will include the text of the field regardless of the sharedText  
property.
The dictionary does not mention this effect of the "sharedText"  
property, saying (what is not strictly true) that "If the field is a  
card field (not part of a group), its sharedText property has no  
effect."  I can add a user-contributed note there, and to the
copy command entry, unless someone determines that my findings are  
incorrect.

David Epstein