Why Unicode will never endorse certain proposals
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By making the Private Use Area "private", the Unicode Consortium imposed on
itself a restriction to stay absolutely neutral on the use of these
characters. In other words, it cannot promote or appear to be promoting the
use of this area for any one *particular* purpose. Nor can the Consortium
endorse, or appear to be endorsing, any particular method of identifying
the repertoire or usage of these characters. Doing so, would change the
nature of the private use area from something that is private and outside
the scope of the Consortium to something that is a formalized code
extension technique.
Why everyone else is free to do what they want
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By definition, this restriction does *not* apply to any other organization
not involved in maintaining the standard. For example, vendors, user
groups, and individuals are quite within their rights to propose particular
assignments or even to define higher level protocols that regulate the use
of the private use area, as long as these apply to *those users, and only
those* that subscribe to that assignment or higher level protocol.
Why certain things may or may not be advisable
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Someone (for example IETF or W3C) who is in the business of defining
general protocols for text interchange built on top of the Unicode Standard
would probably want to be very careful about issues relating to the private
use area. There are three options:
a) The safest thing is to prohibit the use of the private use area
altogether - this maximizes the success of any interchange.
b) In the future, there may be a web-scalable way to characterize the
private use area assignments - in that case they could be built into the
protocols. The interchange would be definite, but at a considerable cost to
everyone.
c) Some protocols may be designed to cover any form of plain-text without
loss. Such protocols would need to allow unrestricted use of the private
use area, but success of interchange would depend on outside negotiation.
Why interchanging private use characters won't work
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Because our growing dependency on internet and web protocols, data
interchange among a community of users who rely on a common set of private
use characters seems hopeless without the existence (and widespread
implementation) of option b. However, if it simply involves the use of a
common font, option c would work as well (with distribution of the common
font being the outside negotiation). Anything more complex would run into
the need to customize editors, browsers, databases etc. in ways that
probably wouldn't be possible or not uniformly successful.
Since option b increases implementation costs for everyone, it is not
likely to be supported everywhere. Therefore, communities that share a well
defined set of characters are better off if they can be standardized.
