I did a search on Google images, and did not find the specific PEX fittings you are asking about. It looks to me that the white plastic tab is just a spacer device to get the installer to place the crimp ring the proper distance from the end of the tubing. A good idea.
I personally use Sharkbite brand fittings for all new PEX work. They don't need a crimping tool, just slide on to the end of the tubing, a compressed O ring inside makes a good enough seal that Sharkbites are allowed by plumbing code to be placed inside walls, and there is even a small removal tool to easily take them off of the tubing, if you so desire. They are more expensive than regular PEX crimp fittings, but I think that the benefits are worth the extra cost.