For [re]synchronization, only the first byte of each command has msb == 1 For 63 of 73 commands. 5 lsb of first byte is an Arduino sketch-specific command: setting e.g. PWM parameters: (frequency, % ...
Say you wrote a command line tool that runs arbitrary javascript (e.g. task runner, test framework, etc). For the sake of discussion, let's pretend it's a testing harness you've named testify.