It's according to how the tree the fruit came from was made. If it is a seedling, it will be true to form. That is the seedlings will be the same. But most are grafted, in which case you will get the root stock seeds. It could also be a hybrid, in which case the fruit would be a blend of the two types used in the cross.
The rootstock for most citrus is trifoliata orange (Sour Orange). It is used because it is more resistant to pests, bacteria and fungal issues. The rootstock has no effect on the fruit the graft produces. The graft will produce viable seeds and if planted will produce whatever fruit the graft bears, just won't have a great rootstock. Unless the rootstock produces sucker shoots that are not trimmed off it and allowed to flower, it has no effect on what is grafted to it.
Edit: As an example let's use walnut trees, there are two common kinds of walnuts, Black Walnuts, which have very hardy rootstock, but produce smaller more bitter walnuts, and there's English Walnuts, which produce larger meatier walnuts, but have a more delicate root system. As a result, English walnuts are grafted onto Black walnut rootstock. The resulting plants produces English walnuts, any shoots from the rootstock are trimmed off. Those walnuts can be planted and will be English walnuts, with English walnut root systems The graft lasts about 40 years, then the English walnut portion dies. When the grafted tree dies, the exposed black walnut rootstock often puts out shoots, and will be a black walnut tree and produce black walnuts, if allowed to mature.
