Hi

domicile state is that state in which you have been living for more than 10 years or it is your permanent resident state .Your home state is your domicile state and in that state you can participate under state quota by showing your domicile certificate because the authorities need a valid proof of you being a permanent resident of that state .Domicile certificate is the most valid proof of you belonging to your domicile state .So , if you want to get admission under state quota of your state then you have to show your domicile certificate of that state .

Suppose you belong to Karnataka then you have to get a domicile certificate by the central or state government to show a proof that you belong to Karnataka and with  that proof you can get a seat under state quota of Karnataka .

Usually the cut off for state quota are less.

Hope it helps!