Let formalize it
Round function $R:\{0,1\}^b \to \{0,1\}^y$ (This is bad naming, $F$ was better here as in DES)
Input to each round is $b+y$ bit register/array $I$.
Output of each round $O = (R(\texttt{MSB}(b,I)) \oplus \texttt{LSB}(y,I)) \mathbin\| \texttt{MSB}(b,I)$
Therefore $O$ is again $b+y$ bits register/array as an input to next round.
- $\texttt{MSB}(b,I)$ the Most Significant $b$ Bits of register $I$.
- $\texttt{LSB}(y,I)$ the Least Significant $y$ Bits of register $I$.
Example
Let $I=\texttt{[0,1,1,1,0,1,0,0,1,0,0,1,1,0,0,1]}$ be 16-bit register then
- $\texttt{MSB}(3,I) = \texttt{[0,1,1]}$, and
- $\texttt{LSB}(12,I) = \texttt{[1,0,1,0,0,1,0,0,1,1,0,0,1]}$
Note that here we used binary representation for $I$, not the array repsentation.