What is the difference between "x", "-" and "0" in Table 9 of the ISO-7816-4 Section 5 documentation

Question

I'm going through the ISO-7816 documentation and having trouble interpreting the CLA scheme under section 5.4.1, Table 9:

b4 b3 b2 b1	Meaning
x x - -	Secure messaging (SM) Format
0 x - -	No SM or SM not according to 1.6
0 0 - -	No SM or no SM indication
0 1 - -	Proprietary SM format

What I understand so far is that if CLA = 8X, then the above nibble represents the various patterns that "X" can take on. What do the symbols "x" (lowercase) and "-" imply in terms of the value of the bit at that position? More concretely, what would a CLA of "80" mean? How is 0000 different from xxxx or ---- ?

Answer 1

More concretely, what would a CLA of "80" mean?

CLA=80 corresponds to the proprietary class, because "Bit b8 set to 1 indicates the proprietary class". Table in section 5.4.1 specifies interindustry class, i.e. where bit 8 set to 0.

What do the symbols "x" (lowercase) and "-" imply in terms of the value of the bit at that position?

You can treat mark 'x' as wildchar (any value), mark '-' as "bit is not used in this case", and 0 and 1 as exact values for bit place. So, xx-- is just a bit mask. It tells that bit 4 and bit 3 indicate what SM format is used int the command and bit 2 and bit 1 are used for something different .
0x-- can be 00-- or 01--.

Bits 2 and 1 are described with an other line of the table.