What is the switch for on my S3 remote?

http://customersupport.tivo.com/LaunchContent.aspx?CID=46ef36f8-62ec-4eca-9937-dcb2fe063fd4

 

It's there to switch between two different TiVo's but you have to program the TiVo's to respond to one or the other.
The TiVo's default is to respond to any remote code.

IIRC, the remote is pre-programmed with codes 1 and 2 respectively but the TiVo defaults to remote code 0 which means they respond to any remote code. So they will both appear to respond the same no matter what you have the remote switched to.

You need to program your TiVos one at a time to respond to only one remote code.
Edit: see link above

Remote code 0 is universal both on the TiVo and the Remote.
So if the TiVo is set to remote code 0, it will respond to any remote code no matter what code the remote is set to.
If the remote is set to remote code 0, any TiVo will respond to it no matter what the TiVos remote code is set to.
In order to control only one tivo at a time you need to switch all tivos and the remotes to something other than 0.

Remember this if you're programming a universal remote or you order a new remote from TiVo.

 

Cheers!