A MIDI Controller has a MIDI Controller that sends MIDI Controllers

MIDI Controllers (Products, Physical Controls, and Messages)

Unfortunately the word controller is overburdened in the MIDI lexicon and probably the most overused word in the world of MIDI. 

It can refer to three different things- products, physical controls and messages. 

MIDI Controller=Product 

People can say MIDI Controller and they mean a product like a IK MultiMedia iRig Keys I/O 25 Controller Keyboard.  

They might say " I'm using The Roland  A88 MK2 as my MIDI Controller".  

MIDI Controller=Physical Control

But the word Controller is also used to refer to physical controls like a Modulation Wheel, a Pitch Bend wheel, a Sustain Pedal, or a Breath Controller (yes, there is that word again).

MIDI Controller=Control Change Messages (Controllers)

The word Controller is also used to describe the MIDI messages that are sent.  So you could say "I'm sending Controller #74 to control Filter Cutoff'. 

In fact, there are multiple types of MIDI messages that are sometimes referred to as "Controllers":

• MIDI 1.0 Control Change Messages
  
• Channel Pressure (aftertouch)
• Polyphonic Key Pressure (poly pressure)
• Pitch Bend
• Registered Parameter Numbers (RPNs) in MIDI 1.0 that equate to the 16,834 Registered Controllers in MIDI 2.0
  
• Non-Registered Parameter Numbers (NRPNs) in MIDI 1.0 that equate to the 16,834 Assignable Controllers in MIDI 2.0
  
• MIDI 2.0 Registered Per-Note Controllers
• MIDI 2.0 Assignable Per-Note Controllers

To make things a bit more convoluted, the MIDI 1.0 specification contains certain MIDI Messages that are named after physical controls specifically- 

Decimal   Hex      Function 

1              0x01    Modulation Wheel or Lever 

2              0x02    Breath Controller

4              0x04   Foot Controller

11            0x0B   Expression Controller 

64            0x40   Damper Pedal on/off (Sustain)

66            0x42   Sostenuto On/Off

67            0x43  Soft Pedal On/Off 

But these are MIDI Control Change (CC) messages, not the actual physical controllers themselves. 

However most products hardwire the Mod Wheel to CC#1 and set the factory default of Damper to be assigned to CC#64, etc.  

Also on most MIDI products you can set your physical controller Mod Wheel to send different CC messages (for example Control Change #2 Breath Controller or Control Change #11 Expression). 

MOD WHEEL is a physical controller that always generates a specific message cc001 Modulation Wheel. cc001 (Control Change) can be applied to most any function, it does not have a fixed function. It is most often used to apply Modulation depth to pitch (vibrato) but that must be assigned to the wheel on a per program basis.

by  Yamaha Product Specialist Phil Clendennin ( AKA Bad Mister)

So a MIDI Controller has a MIDI Controller that sends a MIDI Controller! Or translated into a sentence that makes more sense-

A IK MultiMedia iRig Keys I/O 25 has a Mod Wheel that sends Control Change (CC) #11 Expression. 

The important thing to remember. 

The word MIDI controller can refer to three different things. 

• A type of product- The IK MultiMedia iRig Keys I/O 25 is a MIDI Controller
• A physical control- The Mod Wheel on the IK MultiMedia iRig Keys I/O 25 is a MIDI Controller
• A MIDI Control Change Message- The Mod Wheel on the IK MultiMedia iRig Keys I/O 25 is sending MIDI Controller #11 Expression