What are Butt Weld Fitttings? - Butt Weld Fittings are a family of fittings used in connecting and creating pipe-work systems whereby they are welded into the system using circumferential butt welds.
They are used only in conjunction with ANSI Pipe and are available in the same size range.
They are used in areas where pipe-work is permanent and are designed to provide good flow characteristics.

What manufacturing standard is required and is the fitting seamless or welded?

Wrought pipe fittings are manufactured to dimensions and tolerances in ANSI B16.9 with the exception of short radius elbows and return bends which are made to ANSI B16.28. Light-weight corrosion resistant fittings are made to MSS SP43.

Butt Weld Fittings are available to ASTM A403, ASTM A815 and MSS SP43. These standards require the fittings to be manufactured as follows:

Seamless austenitic fittings are made from seamless pipe to ASTM A312.

Welded fittings in austenitic grades are manufactured from welded pipe to ASTM A312 or plate to ASTM A240. Note that welded fittings manufactured from plate may have two welds.

Duplex (ferritic/austenitic) grades are manufactured from pipe to ASTM A790 or plate to ASTM A240.

ASTM A403/A815 Butt Weld Fittings are sub-divided into four classes:

WP-S: Made from seamless pipe to ASTM A312 (Austenitic) or ASTM A790 (Duplex).

WP-W: Manufactured from welded pipe to ASTM A312 (Austenitic) or ASTM A790 (Duplex). There is no requirement for radiography unless a manufacturer’s weld has been introduced or there are welds made with the addition of filler metal.

WP-WX: Of welded construction. All welds must be 100% radiographed in accordance with Paragraph UW-51 of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code.

WP-WU: Of welded construction. All welds must be 100% examined ultrasonically in accordance with Paragraph UW-51 of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code. Note that this Class only applies to austenitic fittings made to ASTM A403.

CR Fittings are manufactured to the requirements of MSS SP43. These are light-weight fittings and do not require radiography.

Notes:
WP: Means Wrought Pipe
CR: Means Corrosion Resistant

What markings will be on the fittings? - The full identification of the fitting should be marked on it including:
Nominal Pipe Size (Nominal Bore)
Schedule (Wall Thickness)
Specification
Grade
Method of Manufacture (Seamless or Welded)
Heat Number
Manufacturer’s Name or Symbol