The people in semi/shared slots just share the server space but they cannot see what the other person is doing etc, the hard disc space is separate mostly through partition.
The things that are generally shared are :
So keep that in mind while going doe a semi seeedbox, general rule of thumb too many users in a semi plan is not a good idea.
It differs from a dedicated/ vps where you are the only one user and therefore given root access and you have complete control. It is also more expensive and usually not needed if all you want is a seedbox.