Apple Logic Pro 9 - Manual (Page 1256)

This makes playback sound much more realistic and alive.
The default settings in the MIDI Meaning pane (in new projects) are 0 for velocity, and
100% (no change) for length. If you don’t change these values, the symbols remain purely
graphical, and do not affect MIDI playback.
If you record your regions in real time (from a keyboard), it’s better to leave these settings
at the defaults, as the notes probably already sound the way you want them to.
Important: If you do use MIDI Meaning, you need to adjust the settings before you begin
to insert accents and so on. This is because the settings have no influence on accents
and phrasing marks that have already been inserted.
Once set, the velocity and length of notes will be changed as soon as you attach one of
these symbols to a note. When you delete the symbol, note velocity and length are reset
to their initial values.
• Velocity: This value is added to, or subtracted from, the original velocity value of the
corresponding note.
• Length: The original note length is shortened by the percentage chosen here. This only
affects the playback of the note, not the score display. The note length change can be
seen in the Event Parameter box or in the Event List.
1256 Chapter 43 Project Settings in Logic Pro