Is it correct to have such a code where the output depends on three input signals, but the signal C is not relevant in each case? So I do not specify this C in every case.

I mean, for example, is the first case fulfilled when A and B are = 0 and then there will be X1 on Y or is this however treated as the last else and the output will be '0'.

Y <= X1 when (A = '0' and B = '0') else           
     X1 when (A = '0' and B = '1') else           
     X2 when (A = '1' and B = '1' and C = '0') else 
     X1 when (A = '1' and B = '1' and C = '1') else 
     X1 when (A = '1' and B = '0') else           
     '0';