Typically, a siridar, or planetary fief meant that the ruling clan, or Great House, ruled over the entire planet, and was charged with the upholding of Imperial law, as well as the social order imposed by the Faufreluches system. The planetary governor would typically be referred to by adding the word "siridar" in front of his existing title of nobility, and would be required to submit a tithe to the Emperor after collecting profit and taxes from the populace of those living within the fief.
A siridar fief could last for thousands of years. However, it could be withdrawn by the Padishah Emperor at his discretion, or he could order a Great House to abandon one fief and take another.
It seems very clear that the Galach siridar, Fremen al-asiridar derives from the original Arabic as-sadar," i. e. "voice" [of the Emperor], this usage presumably referring to the fact that the designated official speaks in the Emperor's name to stipulate what will happen.
Behind the Scenes
After the death of Leto Atreides II, it is not known whether planetary fiefs continued to exist, especially since there was no emperor to give them. Indeed, it is not known whether Leto II himself allowed the fiefs of the Great Houses to be maintained.
Moreover, it is also not known whether the Great Houses that were re-emerging during the time the Lost Ones were returning from The Scattering were also holders of planetary fiefs, or whether their Great House status was acknowledged because their power developed through other means, such as market manipulation and entrepreneurial activities.