What does those American standard No mean, like API6D,API600,API608,API609….

Commonly used standard number for American Standard Industrial Valves,Flanges, Pipe Fittings:

ANSI B16.5: steel flanges and flange fittings 

ANSI B16.10: face to face length of iron, steel industrial valves 

ANSI B16.11: Forged steel pipe fittings of socket welded and and threaded connection. 

ANSI B16.20: Ring gaskets and grooves for steel pipe flanges

ANSI B16.25:Butt welded connection ends 

ANSI B16.34:steel industrial valves

ASTM A105:carbon steel forgings 

ASTM A126 :Grey iron castings for industrial valves,flanges and fittings

ASTM A182:forged and rolled alloy steel components 

ASTM A193:Alloy steel bolts 

ASTM A194:Carbon steel and alloy steel nuts 

ASTM A216:Carbon steel castings 

ASTM A217:Alloy steel castings 

ASTM A232:Quality standard for chrome vanadium alloy spring wire for industrial valves 

ASTM A276: Stainless heat-resistant steel bar type material

ASTM A320: Low temperature alloy steel bots 

ASTM A350: carbon steel and low alloy steel forgings for pipes 

ASTM A351: stainless steel casting 

ASTM A352: Low temperature carbon steel and alloy steel castings

ASTM A479: alloy steel bars and sections 

ASTM A540: Special alloy steel bolts 

MSS SP25 :Symbols for industrial valves, fittings, flanges and fittings 

API 6D: Specifications for industrial pipeline valves 

API 598: Inspection and testing of industrial valves

API 600 :Flanged or butt welded steel gate valves 

API 602 :small diameter gate valves,globe valves and check valves for oils and gas industry. 

API 609:Double flanged, lug type and wafer type butterfly valves 

API 608: Flanged, threaded and welded metal ball valves