Deciding what kind of thread is used where depends highly upon the object which will be using it and exactly for what purpose. 

The adapter with both end positions can be either male or female. They are responsible for creating a joint which pretty much seals up a joint from one end or both ends. 

Without threads, the sealing and holding aspect of two different entities can become extremely difficult. 

Different kinds of threads have been created for different purposes. From daily life objects to the transportation of volatile materials like fuel and gas, threads are manufactured accordingly.

But there is one type of mechanical thread that is considered a common or basic format in the Metric Adapters.

The common format is divided into two parts the first one is  called the UNC and the second one is called the UNF:

UNC: 

The full form is United Coarse Threads. By the name, we can get the idea of what kind of thread it might be. These are course or unrefined threads. 

These threads have higher durability. Both UNC and UNF have a 60-degree angle which makes the combining process easier. Using them as a force bearer is the best way of using them.

UNF: 

The full form is United Fine Threads. It has a similar angle of 60-degree to the coarse type. But the UNF has a finer quality thread. It is more refined and properly produced. 

Therefore, it is more suitable for sealing procedures as this will ensure a toral vacuum from the thread end.