\subsection{SISO \& MIMO}
\bi{S{\scriptsize ingle-}I{\scriptsize nput-}S{\scriptsize ingle-}O{\scriptsize utput}}: $r$ Reference (e.g. speed), $u$ Sys. Input (e.g. gas pedal pos),
$z$ Sys. Out. (e.g. speed of car)

\bi{M{\scriptsize ultiple-}I{\scriptsize nput-}M{\scriptsize ultiple-}O{\scriptsize utput}}: $r$ Reference (typ: trajectory), $u$ Sys. Input (e.g. 4 rotor speeds),
$x$ internal states (pos, orient, speed, rot. speed), $z$ Sys. Output (e.g. speed of car)