Cipher Type Byte.
See Section 4.2 of RFC 4880 for more details.
OpenPGP defines two packet formats: the old and the new format. They both include the packet's so-called tag.
The new CTB format.
The old CTB format.
A sum type for the different CTB variants.
The PacketLengthType is used as part of the old CTB, and is partially used to determine the packet's size.