Symmetric is a system comprised of an ingeniously simple concentric method of drilling through overburden with casing
Eccentric is the most economical solutions because its ingenious reaming wing the bit is retrievable can be used at the next hole
Adapters can be used to adapt drill rods with different types of thread, or connected to the drill powerhead to protect its thread.
The DTH is the abbreviation of Down The Hole and sometimes it is also called RAB in some markets, then the RAB is short for Rotary Air Blasting