The protocol specifies a specific set of control bytes, which are listed below. The EvenParity apply() serves both to document the required parity and ensure that parity is appropriately set.
It might be argued the above design is too simplistic since it does not use any typing mechanism to specify the concept of a control character. That additional sophistication (and complexity) is not useful for this problem domain.
Some of the control characters are used to control transmission (details to follow). The PairByte object provides an extractor that detects those special characters. (The nomenclature arises because these characters appear in pairs)