By default, a layer has the same name as its source footage item. If you toggle between viewing the layer name and source name, you'll see that the layer name has brackets around it to distinguish it from the source name. If you change the layer name to a non-default value, the brackets go away.
Is that what you were referring to?
Not exactly. The brackets come and go even if I don't change the layer
name to something other than the source asset. And the rectangles around
the layer names also appear and disappear. I think all this has
something to do with applying effects. But...I don't see a pattern. I
think the rectangle is just to let you know which layer is being
changed. But, again, I'm not sure of that.
The rectangle indicates selection state, as shown here.
Toggle between viewing the layer name and source name in the Timeline panel and see if that makes it clear to you what is going on with the brackets.
The second link explains how to switch from the Layer Name to the Source
Name view in the Timeline panel. It doesn't state that when the Layer
name is the same as the Source name, brackets appear around the name in
the Layer Name view. But...now that I've switched back and forth between
the two views, I see that's what's going on.
I added a comment about this in the Help file.