What is it?
Okay lets see if we can answer this question once and for all.
In piping we all know we have two basic situations; one is Fitting-to-Fitting and the other is not Fitting-to-Fitting.
With the first, there is nothing between one fitting and another except the weld. Therefore we call it "Fitting-to-Fitting or "Fitting make-up"
With the second, the required dimensions do not allow for Fitting-to-Fitting configuration. We then have an ELL, gap, ELL or ELL, gap, Reducer, ELL or ELL, gap, Flange or ELL,gap, reducer, flange or some combination of standard fittings BUT not Fitting-to-Fitting. There is that short gap between two of the fittings.
This short gap is where a piece of pipe must be placed (cut, beveled, fitted and welded) to connect the two fittings that don't touch. The short piece of pipe is called a "Pup" piece. The piece of pipe or "Pup" can be pretty much any length however when it gets to be around a foot in length it is no longer called a "Pup". A long piece of pipe is just pipe.
The only restriction for a"Pup" piece is common sense or a company imposed minimum length. This minimum length is defined in different ways by a Lead Design Supervisor or company and there is no absolute restriction or commonality. Individual or company minimums sometimes consider the pipe wall thickness, the weld bevel, the heat effected zone, or the precieved ability of the shop to make a weld. There is no magic rule and there is no common industry standard.
A "Pup" piece is just a short piece of pipe cut to fit and fill the gap between two fittings when "Fitting Make-up" is not possible.
Do it once and Do it Right 
