The mailing list for listmasters using Sympa

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I find the Help to be insufficiently helpful.  I found no examples in the 
docs.

Regarding the sympa List Administration Panel:

Custom Header Field
   Is there a particular format I should when specifying a Custom Header Field
for a sympa list?

   What will the header look like? Is it created as an X- header?

Subject Tagging
   Is this simply a literal string that will be used as a prefix to the
subject?
   Will spaces be added or must I include those? 
   Will a : be added or must I include that?