TElPGPPrimaryUserIDSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 188.8.131.52.:
« This is a flag in a user id's self signature that states whether this user id is the main user id for this key. It is reasonable for an implementation to resolve ambiguities in preferences, etc. by referring to the primary user id. If this flag is absent, its value is zero. If more than one user id in a key is marked as primary, the implementation may resolve the ambiguity in any way it sees fit.»