Above is the midi sequencer window. Propellerhead Reason
Sequencer tracks are on the left had side and midi events are in the main view.
The top editing bar is along the top. With the ability to toggle screens then followed by editing functions and finally. Musically editing events.
The above shot is of timeline events.

Above the toggle buttons allow you the options of moving between these screens where you can edit hits. The velocity of the hit and note events in the piano roll editor also.

An overdub is a recording over an existing sequencer section/audio wav. The result is dependant on the mode setting on sequence transport bars. You can merge the new recording or simple erase the recording. When recording overlapping parts the Rec Mode setting on the Transport panel in Cubase Sequencer needs to be set. "Normal", overdub recording, records over audio. "Merge", the events are added to the existing part. This is relevant in midi recordings "Replace", recording replaces any existing events when two events collide. The mixdown gives you an option as to what format to save the file. You also have a choice of channels Mono, Stereo and Stereo Interleaved etc.