I will go for Internal Medicine Sub-I at University in the South.
I heard that Sub-I differs than visiting elective in that we work like residents.
While the level of responsibility and autonomy is increased on a Sub-I, you are not a resident. It is obviously variable between location, residents, and you as to how much responsibility you have. However, there is really very little difference between an away and an away Sub-I, except that by labeling "Sub-I" you effectively communicate "I want to work harder, and do more stuff."
More important than it "being a Sub-I" is that you set your desires and expectations up front, at the start of the rotation. Say, out loud, "I want to do more than just some passing by medical student. I want responsibility, I want procedures, but I will need your help."
That tells them you are serious about the rotation, and you are willing to do more.
Are we allowed to write orders, prescribe medications & sign on papers ?
None of the above. You are a medical student. Likely, you will write the order (on paper or in the computer) while your resident supervises you, and, if need be, signs off on your order form. You can sign all you want, but your signature means nothing; everything needs to be signed by a physician.
Or the residents will do that & we just tell our plans ?
It is variable, dependent on the rotation, the resident, and you. If the resident is comfortable with your abilities and their own knowledge, you may get a bit of leeway with what you are allowed to do. Show poor knowledge or weakness, or worse, you get an incompetent resident who isn't ready to lead, and you may be restrained. This is what makes Sub-Is as an away so challenging... to have a good experience all things need to align.
What are tasks that we are allowed to do/are expected to be done ?
Most places have number requirements for their residents. So unless it is a particularly busy hospital (so the residents already have their procedures logged), the interns and the residents are going to do the procedures (central lines, paracentesis, thoracentesis, intubations). You will likely be making medical decisions, rather than doing actual procedures. If you want to get some exposure, make sure you speak up. Say out loud, before the procedure is prepped, that you want to do it. If they say OK, you're in. If they say no, then accept it.
What are the mistakes that I have not to do in order to avoid liability insurance issues?
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If you are an FMG, you need to get your own liability insurance. Its cheap and easy to come by. No offense meant to you, but from the grammar of your post, you sound like you might be. There is a thread already in place here at SDN that tells you some options.
I hope the message came through.
The Sub-I is highly variable dependent on a number of things. The most important is that you
communicate your (lack of) desire for procedures and responsibility up front, so they know what to expect from you, and they can give you their response.