Choose this if you have, for instance, checked the ID card of the person, and have verified that they are in possession of the secret key (e. g. by sending them encrypted mail and asking for it to be decrypted).
Escolla isto se, por exemplo, comprobou o cartón de identidade desa persoa e verificou que posúe a chave secreta (p. ex., enviándolle mensaxes cifradas e pedíndolle que as descifre).