Apologies if this is a dupe...
I always get this wrong when I'm not thinking, but a field is a single
(logical) line in the header of a message; the header is the entire
set of fields -- a message has only one header.
In all the documents, there's a general need to do
s/header/field/
However in some cases 'header field' may be necessary to give context.
Also there's a need to spot the cases where 'headers' is used to mean
the entire header of the message, where it should be replaced by
'header' rather than 'fields', eg in section 1 of -pra 'the headers of
the message' should be 'the header of the message'
-roy