About Trim Events to Part
Cubase VST works differently from MIDI when it comes to handling notes. Where MIDI regards Note Ons and Note Offs as separate entities, Cubase VST stores notes, their position and length. This means that even if you change the length of a Part by using the Pencil or Scissors tool you may get notes that play past the end of the Part. This is a valuable feature, but you may not always want this to be the case. If you don't, you should use Cut Events on the Part(s).
Performing Trim Events to Part
1. Select the Part(s) that are to have their notes Trimmed.
2. Select Trim Events to Part from the Structure menu.