TElPGPExportableCertificationSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 18.104.22.168.:
«(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.»