Meaning of $u(t)$ in $x'(t)=f(x(t),u(t))$?

Question

For a first order ODE we have the forms

\begin{align} x'(t)&=f(x(t)) \\ x'(t)&=f(t,x(t)) \end{align}

However, I have also seen ODE:s with a second function $u$, i.e.

\begin{align} x'(t)&=f(x(t),u(t)) \\ x'(t)&=f(t,x(t),u(t)) \end{align}

Wwhat is the meaning of $u$ in the function $f$?

Thanks!