I just purchased my first carto tanks, 2 SmokTech 510 UDCT V2 3.5ML tanks. After figuring out how to prime the carto and fill the small space between the tank and carto with e liquid, I noticed the tank came with an extra white rubber seal as shown in the picture. What is this for?? The only thing I can thing of is to put it directly into the carto between it and the drip tip. I didn't do this however because I am concerned it would obviously melt. Is it to prevent liquid from escaping through the drip tip or is it something else altogether? Thx!