Hi Maystro. Check this out, from an engineering site.
Application of UNC (Unified coarse) thread
Coarse thread series (M/UNC/UNRC) is generally used for mass production of threaded fasteners, such as bolts, screws, nuts, etc. It has unique advantages in some rough operation applications that require quick assembly or disassembly and are easy to produce corrosion and damage.
Application of UNF(Unified fine) thread
The UNF thread has smaller pitch than the UNC thread with the same nominal specification, higher thread strength, smaller space occupation size and better self-locking performance. Fine thread series (MF/UNF/UNRF) is generally not used for large stress work, its general uses are applications need high fastener strength. It is mainly used for positioning, adjustment, fixation, etc. In addition, fine thread can be used for fine adjustment. For example, lens fixing of various optical instruments, micrometer and other instruments used for measuring, threaded connectors, battery cover of metal shell flashlight, etc. However, the height of the fine thread is small, so it is easy to get disordered when installing, and the burr has a great influence on the assembly, so it must be cleaned up. Additionally, the removal times of fine thread should not be too much.