How does the contract of *Ja'alah* differ from *Ijarah*?

Primary text

*Ja'alah* differs from *Ijarah* in that *Ja'alah* is a revocable contract (*Ja'iz*), whereas *Ijarah* is binding (*Lazimah*). Furthermore, in *Ja'alah*, knowledge of the duration, the exact measure of the work, and the conclusion of the contract with one specific designated person are not considered requirements.