The Meaning Behind White Arrow/Grey Area in Compound Clips
I've searched the forum to no satisfactory answer, so I'd try to explain this way.
I've been doing some changes to projects w/ compound clips and assigned roles - mainly replacing the imagery while retaining all other properties (especially the duration of each timeline).
And I notice from time to time when I double click the compound clip to edit, there is this white arrow in the timeline, and the area that is greyed out, in what seems to be an arbitrary and random place, but I can still edit those clips in the greyed out area (for example, replace an image).
Below are two screen shots of the compound clip and the very same source clips in the timeline, with arrows and greyed out areas.
iMac, OS X Yosemite (10.10.4)