Just the 993 expansion set. You can add as many remotes as you like as long as you only have 1 990 base connected to the layout .
I.e. friends can bring their handhelds over. They will automatically pick up the locos stored in your base.
The 993 is merely a charging station. It has no control capability.
I believe the only stipulations are the remote(s) and base need to have the same software version loaded, and be on the same channel, which is selectable.
On a side note. You can use the handheld from a second 990, just not the base.
