Here's a longer reply-- hopefully not too boring or confusing! I'll start with a general discussion before touching on MIDI device setup in Studio One.
As you may know, a MIDI port can either send (Out/Thru) or receive (In) up to 16 channels of MIDI data.
The MIDI device that's sending/receiving the data might not be able to use all 16 channels at once, as that depends on its capabilities. For instance, an analog synth might be able to play only one patch at a time, so it would use only one channel at a time. A MIDI keyboard controller that can be split or layered might be able to use three channels at once for sending Note events. But many devices can indeed use all 16 channels at once.
In fact, some devices might have multiple sets of MIDI In/Out ports, allowing them to send/receive more than 16 channels of data-- 32, 48, 64, etc. (i.e., a multiple of 16). Most DAW software programs can open several MIDI In/Out ports at once, so they can use more than 16 channels, although each port is limited to 16 channels.
The 16 MIDI channels are not "universal." The 16 channels of one port are different-- separate and distinct-- from the 16 channels of another port. Thus, it's possible to be using channel 1 of one port for one thing while simultaneously using channel 1 of another port for something else without needing to worry about them interfering with each other--
although that depends on the situation. (Just because something is "possible" doesn't mean it's always the case.)
So when do you need to worry about this? If you have several MIDI devices connected together in a daisy chain, then each device in the chain is receiving the same 16 channels of information from the device at the head of the chain. In that case you do need to worry about which device is using which channel(s), since you presumably want each device to use different channels than the other devices, otherwise you could end up with all of the devices playing the same thing!
But when you have two or more devices connected to a computer and each one has its own MIDI In/Out ports in a DAW, the channels of one device will be distinct from the channels of the other device(s). Thus, it's okay for you to use the pads on an Alesis VI25 to trigger drum sounds in the Steven Slate software, and at the same time use a KAT Multipad to trigger different drum sounds in the same software.
Furthermore, most DAWs have multiple functionalities-- i.e., they can act as recorders, mixers, sequencers, routers, etc. If you need to, you can set up a track in Studio One to receive channel 1 from the VI25 and send that data to the Steven Slate software on channel 2, and set up another track to receive channel 1 from the Multipad and send that data to the Steven Slate software on channel 3, or something like that. In other words, a track's incoming channel number doesn't need to match its outgoing channel number, and it's okay for two different tracks to use the same incoming channel number-- or, for that matter, the same outgoing channel number-- without interfering with each other if those channels are coming from (or going to) different devices.
Different DAWs may have different ways of handling MIDI devices and ports. In some DAWs you can simply connect a MIDI device and start using it, choosing at will which MIDI channel(s) you want to use with that device. But in Studio One you need to "add" a device-- its In/Out ports will be visible as soon as you connect the device to the computer, but you must associate its ports with a device setup. This has advantages and disadvantages. The main disadvantage is that it's a bit of a hassle to have to set up every device before you can use it, especially if you need to set up the same device multiple times in order to accomplish what you're wanting to do. But the advantage is that you have flexibility in how you can set up each device.
Studio One lets you create three different types of device setups-- a "New Keyboard," a "New Instrument," or a "New Control Surface." The "New Keyboard" option is used to set up the MIDI input that Studio One will receive from a device, so it's the type you're interested in.
When you add a new keyboard, you can specify which MIDI channels you want to receive from it. The default setting is for all 16 channels to be enabled, but you can deselect any channels you don't want to receive. You can also filter out certain types of MIDI events from the incoming data. In most cases you'll probably want to accept the default settings-- i.e., receive all channels and all event types.
But there may also be times when you'll want to receive only one channel at a time, or filter out certain types of events. You can create additional keyboard setups for handling those situations.
Note that you can create multiple setups for the same device (i.e., same port), in which case you'll probably want to give each setup a name that makes it easy for you to identify which is which. Studio One will warn you that you've already got a device setup that uses the port, but you can just dismiss the warning.
For instance, I have a Yamaha YPT-400 portable keyboard, which shows up as "Yamaha Portatone-1" in the list of available ports. Excluding the use of styles or "auto accompaniment," I can play up to three different "voices" or instrument sounds on it at the same time-- a Main Voice, Dual Voice, and Split Voice, which the keyboard transmits on channels 1, 2, and 3, respectively. Suppose I want to record the data from each of those three voices to separate tracks. To accomplish this, I could create three separate keyboard setups:
Device Name = YPT-400 Main Voice (or maybe something else, e.g., YPT-400 Channel 1)
MIDI Channels = 1 (i.e., deselect all other channels)
Receive From = Yamaha Portatone-1
Send To = None
Device Name = YPT-400 Dual Voice
MIDI Channels = 2
Receive From = Yamaha Portatone-1
Send To = None
Device Name = YPT-400 Split Voice
MIDI Channels = 3
Receive From = Yamaha Portatone-1
Send To = None
If I add a new track and assign the "YPT-400 Main Voice" device to the track's Input, that track will receive channel 1-- and only channel 1-- from the Yamaha Portatone-1 port. Note that the device will be listed in the track's Input field as "YPT-400 Main Voice | Any" (meaning any channel), but the track will nevertheless receive only channel 1, since that's the only channel which was selected in the device setup.
The "New Instrument" option is used to set up a device for receiving data from Studio One. You won't need to use this option for what you're wanting to do. However, if I wanted to use Studio One to play back a MIDI recording and send the data to my YPT-400 (i.e., to make the YPT-400 play the song like a player piano), I would add at least one "New Instrument" setup for the YPT-400, as follows:
Device Name = YPT-400
Receive From = None
Send To = Yamaha Portatone-1
MIDI Channels = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16
I know this was a lot to read, but I hope it's been helpful to you. Let me know if you have any further questions.
