Carb caps were first commercially released in 2013. They were used on both Titanium and Quartz dab nails. The main function of a carb cab is to provide the smoker with a low temperature dabbing experience. Most carb caps have restricted air flow through them. By taking a low temp dab with a carb cap you will experience a much more flavorful extract dab. Since 2013 the evolution of caps has progressed greatly. When originally released the main purpose was to greatly reduce the airflow going pass through the nail. Holes for airflow quickly followed and were added to most designs.
In 2017 Titanium nails have become much less popular and Quartz has taken a dominating stance in dabbing culture and is becoming the prefered method of most dabbers. Directional carb caps have become much more popular as well. Be allowing direction airflow into the cup of the nail you can swirl your dab around and ensure you can take a lower temp dab and still vap off most of the dab without wasting to much.
Most nails made of quartz can use a traditional carb cap and direction cap while some more unique designs like the thermal wall design created by Pukin Beagle work better with a bubble cap that is another form of direction cap.
Below are some of the more common types of carb caps on the market today.
Also here is a video of us harvesting a carb cap in our garden.... Only in Oregon :)