spec utr50 control

Control Codes, Private Use, and Combining Marks

General Category	Stack	Mixed	Memo
Other, Control (Cc)	R	R	❓ undefined in stacked mode
Other, Format (Cf)	R	R	❓ undefined in stacked mode
Other, Private Use (Co)	U	U	Bias for East Asian use, since other usage is unknown
Other, Surrogate (Cs)	R	R	❓ no need to define?
M*	Follows grapheme cluster