What does "2 digit BCD number" mean?
The idea here is that you can treat a BCD digit as a single entity. Look at the components you have:


1-digit adder
Multiplexer (presumably for BCD digits)
9's complement unit


All of these use full BCD digits for their inputs and outputs, so you don't have to think about the individual bits. \$C\$ and \$ADD\$ are binary, but they never combine with the BCD signals -- they would only be used for the multiplexer selection signals.

So your circuit's inputs are:


\$C\$, a binary signal
\$ADD\$, a binary signal
\$A\$, a two-digit BCD number with digits \$A_0\$ and \$A_1\$
\$B\$, a two-digit BCD number with digits \$B_0\$ and \$B_1\$


And the outputs are:


\$S\$, a collection of four BCD digits, which could be either:


A single number that represents the sum or difference of the inputs, with digits \$S_0\$ through \$S_4\$.
Two numbers which represent the complements of the two inputs, with \$S_1S_0 = \mathrm{complement}(A_1A_0)\$ and \$S_3S_2 = \mathrm{complement}(B_1B_0)\$.