While, The Seating Nipples is a complete different product. It is manufacture of hole of Bar with same threads on both thing but the ID and OD are totally  different at the standard Tubing Nipples, a Seating Nipples is use in conjunction with surface sucker rod Pumps as per N11-20/25 (API Spec 11AX).

 

A Standard Tubing Nipple is a short piece of Tubing with thread on both ends, it is manufactured by cutting tubing to the length of the Nipple desired and then thread both ends. The Thickness of the Tubing Nipple is the same of the Tubing. 

 

The seating Nipple is Manufactured from Hollow Round Bar of steel because wall thickness is heavy much thicker than standard Tubing. 

 

The API that governs the O.D & I.D of the Tubing  and Tubing nipples is totally  different from the API Standard that governs the O.D & I.D of a Seating  Nipple.

 

Another, There is no Standard length for N11-20 or N11-25 seating nipples,  the Length is a Function of Preference by the customer need. It can be 6inch, 8inch, 12 inch, etc..