Most composites shrink so much and with so much force/stress that they'll literally pull enamel crystals off the margin. There's only a couple that truly have low enough shrinkage stress that we don't have to worry about this damage. Exquisite Restoration (Apex Dental Materials, Inc.) is the best of the ones available.
The white lines we see at margins are the result of this shrinkage stress damage. We can't change the amount that the composite will shrink, but we can change the rate of shrinkage, allowing it to release that stress slowly enough to prevent damaging the margin. We do that by adding light a little at a time, and that's called "Pulse Curing". If you're using a composite other than Exquisite Restoration, we highly recommend pulse curing any layer that is in contact with enamel.
For composites other than Exquisite Restoration, I recommend you take the extra time to go thru the pulse curing protocol to prevent damaging the enamel margins. We've previously shown curing bulk fill Exquisite Restoration with the Valo light set on XTRA Power for only 3 seconds and not developing white lines. If you have a Valo light and want to turn up the power to really shorten down the curing time that's great with Exquisite Restoration composite, but probably not for others yet.