TElPGPTrustSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 188.8.131.52.:
«(1 octet "level" (depth), 1 octet of trust amount)
Signer asserts that the key is not only valid, but also trustworthy, at the specified level. Level 0 has the same meaning as an ordinary validity signature. Level 1 means that the signed key is asserted to be a valid trusted introducer, with the 2nd octet of the body specifying the degree of trust. Level 2 means that the signed key is asserted to be trusted to issue level 1 trust signatures, i.e. that it is a "meta introducer". Generally, a level n trust signature asserts that a key is trusted to issue level n-1 trust signatures. The trust amount is in a range from 0-255, interpreted such that values less than 120 indicate partial trust and values of 120 or greater indicate complete trust. »