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Description
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 5.2.3.10.:
«(4 octet time field)
This subpacket denotes whether a certification signature is
"exportable", to be used by other users than the signature's issuer.
The packet body contains a boolean flag indicating whether the
signature is exportable. If this packet is not present, the
certification is exportable; it is equivalent to a flag containing a 1.
Non-exportable, or "local", certifications are signatures made by a
user to mark a key as valid within that user's implementation only.
Thus, when an implementation prepares a user's copy of a key for
transport to another user (this is the process of "exporting" the
key), any local certification signatures are deleted from the key.
The receiver of a transported key "imports" it, and likewise trims
any local certifications. In normal operation, there won't be any,
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
Some implementations do not represent the interest of a single user
(for example, a key server). Such implementations always trim local
certifications from any key they handle.»
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