Linear cryptanalysis and number of linear approximations
It most likely assumes both $a$ and $b$ are nonzero in
$$
a \cdot x + b\cdot y.
$$
If either are zero, the overall inear approximation will be insensitive to changes in plaintext (respectively ciphertext) making the overal linear expresion useless to mount a linear attack on a multiround cipher, since the idea is to extract subkey bits by using enough known ciphertext plaintext pairs.