What is this and why is it important?

MIDI instrument definitions translate program change commands into meaningful instrument names--piano, bass, trumpet, etc.  It is common for MIDI sequencers to provide pre-defined instruments for popular MIDI synthesizers.  Drum note names are particularly useful for editing drum note sequences.

Even with predefined instruments definitions there will nonetheless be many cases where users will have a synth which is not pre-defined, so there should be a facility to easily create instrument definitions.

The sequencer should automatically transmit program change commands for each MIDI track at the beginning of playback.

System exclusive data is part of the General MIDI standard for device-level communication with synthesizers.  A common usage is receiving or sending a memory dump from a synthesizer.  You also need to be able to store and retrieve SysEx data on your computer so an import/export function is necessary.  The best SysEx functions also have editors for SysEx data.

Analogous to an audio mixer, functions to insert FX into a MIDI channel or replicate and send data to another channel are useful.

What is this and why is it important?

My times synthesizers transmit MIDI data that you may not want to receive in your music production software.  Aftertouch and pitchbend controllers are common examples.  Thus it is very useful to be able to filter which MIDI event types you wish to receive.

MIDI tracks typically have several sliders, volume and pan for example, that will send MIDI controller events when they are changed or when playback is initiated.,

MIDI tracks also typically provide for a number of automation envelopes that will send MIDI controller and event commands as they are changed.  Note that these controller events are not included in the MIDI clips and thus cannot be saved independently of the project.

There are a number of note transformations that can be made in real time during MIDI recording and playback.  Transpose, change velocity, and filters are a few examples.

It is reasonable to expect sophisticated selection functions for MIDI events--for example, "all notes between C4 and C6 with a velocity of less than 100".

Over the years sequencers have implemented a whole host of batch processing functions for manipulating MIDI data--transposing, velocity scaling, quantizing, etc.  Thus you MIDI sequencer should provide a good array of these functions.

Complex processing refers to capability of applying programmatic logic to a series of events--"if a note is above C4, then generate a first-inversion chord with this note as the root", for example.

Conceptually MIDI FX have the same function as audio FX, except that they generate additional MIDI events to send to your synth instead of audio.  Thus you can create echo, chords, arpeggios, etc.

What is this and why is it important?

An Event Editor simply lists the MIDI events and allows you to change the values.  Practically all sequencers have this editor, and it is typically used for troubleshooting, not for composing or arranging.

Editing notes using the piano roll paradigm is the most basic function of a Piano Roll Editor, and most sequencers provide this function.

A Piano Roll Editor should also have the capability of displaying and editing note velocity and well as the controllers for a track, and most sequencers support this function.

It is common to have two piano roll editors, one as a separate window which has the most functionality and another which opens on the track in the timeline.  The latter typically has less functionality but has the advantage of quick access as well as being able to see your MIDI events in relation to other tracks on the timeline.

Many people prefer to work with MIDI events using music staff notation.  There is a wide range of functionality and sophistication in staff notation which is beyond the scope of this analysis.  Typically people who prepare professional scores for publishing use separate products such as Coda Finale or Sibelius.