Opaque data type

I'm a bit confused by the "opaque" data type.  I've searched the
archives and have seen the data type used as both input to triggers and
as output from triggers.  At first I thought it meant "nothing" but
after seeing trigger definitions that use multiple instances of the
opaque data type as input, it seems to me that it means something like
"something, but I don't know what just now".

Could someone clarify the concept that underlies the opaque data type?

Thanks.

--
John Burski
I.T. Manager and Systems Administration
911EP
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