This completely depends on the details in your contract. You should check the fine print of what you signed when starting the expat.
For example, I had in my contract (I would assume this is typical, but I have no real data base to say so):
- You are guaranteed a position equivalent to the one you had when you left
- you are guaranteed a salary equivalent to the one you had when you left, plus the customary increases.
- you are not guaranteed the salary, salary increases, or position you reached in the foreign country.
That doesn't sound too great, but in reality, most companies value the experience of the expat time high, and move homecoming people in a higher position (managerial, or if you were a manager when starting, senior management). There is no such promise or guarantee, of course. You are a valuable asset for the company, and they should be interested in putting you in a position of responsibility.
However, what really happens, depends a lot on your respective home-country manager / mentor and his intentions. If he doesn't care (or doesn't like you), it can end ugly. If you are afraid of that, I recommend to get in contact with as many senior management members in the home country as you can - anyone that knows you personally - and ask if they have a position to offer you. Most of them probably value what you bring, and would try to find a nice spot for you if they can. Don't rely exclusively on the manager whos job it should be to care about you.
Note that the previous paragraph assumes that you are a good performer. If you are a low performer (or if people think so), you might end up with the guaranteed minimum and nothing else.