TElPGPPreferredSymmetricAlgorithmsSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 126.96.36.199.:
«Symmetric algorithm numbers that indicate which algorithms the key holder prefers to use. The subpacket body is an ordered list of octets with the most preferred listed first. It is assumed that only algorithms listed are supported by the recipient's software.»