For various practical considerations, it sometimes happens that the codes
making up a text, written in a particular charset, cannot simply be put
out in a file one after another without creating problems or breaking
other things. Sometimes, 8-bit codes cannot be written on a 7-bit medium,
variable length codes need kind of envelopes, newlines require special
treatment, etc. We sometimes have to apply surfaces to a stream
of codes, which surfaces are kind of tricks used to fit the charset into
those practical constraints. Moreover, similar surfaces or tricks may
be useful for many unrelated charsets, and many surfaces can be used at
once over a single charset.
So, recode has machinery to describe a combination of a charset with
surfaces used over it in a file. We would use the expression pure
charset for referring to a charset free of any surface, that is, the
conceptual association between integer codes and character intents.
It is not always clear if some transformation will yield a charset or a
surface, especially for those transformations which are only meaningful
over a single charset. The recode library is not overly picky as
identifying surfaces as such: when it is practical to consider a specialised
surface as if it were a charset, this is preferred, and done.
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