Part cues are a method of adding different timing for different groups of fixtures to use in a single cue. Generally, you build a cue which is a look on stage and then you decide how you are going to get from one cue to another. The Blue Box allows you to change timing of different attribute families without using parts, but it does it for all fixtures in the cue. For example, using the different columns in the cue list, you can have separate times for fixtures going up in intensity than those going down. You can also change times for Position fades vs. Color fades etc.
If you want one group of fixtures to move at a different rate than others in the same cue, you can move those fixtures to a different part.
Note:
You may also want to use the alternate method called Independent Timing where each and every attribute of every fixture can have its own wait and fade time. This is handy if you want staggered offsets and are not concerned about managing parts in the cue list.
You can assign parts either at record time, or when updating the cue.
Select the fixtures that will be in the main part and set their levels. This may include applying palettes or manually adjusting attributes. These fixtures need no further attention before recording the cue.
Select the fixtures for the next part and adjust their levels.
Press the [S11]Cue Parts button to open the cue part dialog box.

Select the button for the desired part.
Note:
The main cue part is labeled "Part A"
Cues need not be made up of parts in specific order. You can have a cue that has just a main part and a Part G.
Press [S12] and roll the wheel to select parts greater than Part K then press [ENTER] to close the dialog box.
If you select any part between A and K, the dialog box will close automatically.
The Selected Fixtures will remain selected but will now show a superscript letter showing the part:

Continue with Steps 2 through 5 until you have built the cue.
Record the cue.
Use the Blue Box to alter timing on the main part or any additional parts.

Goto the cue you want to alter.
Select the fixtures that need to be assigned to a different part.
Press the [S11]Cue Parts button to open the cue part dialog box.

Select the button for the desired part.
The fixtures will show a superscript letter showing the part:

Continue with Steps 2 through 5 until you have assigned all fixtures to their correct part.
Press [UPDT] [ENTER].
Use the Blue Box to alter timing on the main part or any additional parts.

See Also:
Independent Timing
Deleting Cues (including Cue Parts)
Tracking