What does the qualifier NOMINAL exactly refer to? This question may not specifically belong to the physics domain, and rather perhaps to some engineering one, yet I couldn't find a better place to ask it either, trying my chance here...
From time to time, especially during experimentation, calculation, or simulation related research, I come accross the term nominal used to qualify an attribute of a parameter, that I can hardly identify which...
For example, recently I was reading a web page on pipe stress calculation, which, in a calculator, asked me about three parameters, in exact words:

*

*1-) Pressure

*2-) Diameter

*3-) Nominal thickness ???

Now, I don't have an absolutely clear idea of what this nominal thickness may refer to:

*

*standard thickness

*labelled thickness

*stated thickness

*target thickness

*approximate thickness

*average thickness

*expected thickness

*assumed thickness

*design thickness

*production thickness

*measured thickness

*actual thickness

*simulation thickness

*common thickness

*general thickness  (for fun)

As if that was not confusing enough, as the second part of the question, I'm also confused by the use of the term nominal only at the third parameter (the thickness) and not on the Diamater for example, somehow implying that only the thickness can possess a nominal value while the other two cannot?
Since this is just an example case, feel free to replace the parameter thickness with any suitable physical, engineering, or even social science related parameter you may like. Almost always the same confusion happens on what exactly the term nominal refers to.
Any clarifications will be greatly appreciated,
and, please regard the second part of the question as well, and also state if the term was possibly misused on that web page, or was unnecessary to use.
 A: "Nominal" value in these sorts of engineering contexts means the following. In engineering contexts, we often label things by their values. A great example is a 2x4 piece of lumber. 2 refers to the width of the board in inches, 4 refers to the height of the board in inches. However, the actual dimensions of a 2x4 board are approximately 1.5" by 3.5". But, probably because of historical reasons, and because it's much faster to say 2x4, we call them "two by fours"s rather than "one point five by three point fives". In this case "2" and "4" refer to the "nominal" dimensions of the boards. That is, we use 2 and 4 to NAME the object.
But with this comes the caveat that the nominal dimensions may not equal the actual dimensions. Now it's obvious to understand how this can lead to confusion, but it also leads to convenience. Sometimes the nominal value will equal the actual value, but that's not always the case. And that's the biggest lesson to takeaway. If you need to know the ACTUAL value of some dimension then do not rely on the NOMINAL value.
Any parameter can be expressed with nominal values. For example, in atomic physics labs we have microwave frequency generates which generate tones tuned to address atomic transitions. For Rb the frequency we use a tone with 6.834 682 610 GHz, but colloquially in the lab we may refer to this as the 6 GHz tone, the 7 GHz tone, or the 6.8 GHz tone depending on how precise we feel about being with our language. In this case the nominal frequency would be 6 GHz, 7 GHz, or 6.8 GHz. There are no other tones in this vicinity so there's no ambiguity regardless of which "nominal" value is used.
It's a typical pattern that the nominal value is a rounded version of the actual value, but this is not always a great rule to follow. I've had the most confusion with nominal vs. actual values when it comes to different types of pipe fitting such as SwageLok. In this case it seems like the nominal value didn't really refer to ANY dimension of the fitting and I had to look at complex tables to figure out which pieces would actually fit together.
This answer is not an endorsement for using nominal values in a confusing way because that can be bad and slow down work. But it's not an endorsement for NEVER using nominal values either, because it is regularly convenient to specify nominal instead of actual values.
edit:
The question asks why sometimes values may be specific with or without the "nominal" qualification. I'll address the specific example in the question. Some calculation for pipe stress requests the following quantities:

*

*Pressure

*Diameter

*Nominal Thickness
Why nominal thickness but not nominal pressure or diameter? Again context is critical. For example, if this calculator appears on the webpage for a particular manufacturer or pipes it may be the case that (1) users of the pipes may run any pressure through the pipe (2) customers of the pipes can order custom diameters, BUT (3) the pipes are made from some sort of stock material which only comes in fixed thicknesses which the company enumerates as 1/16", 3/32" etc. Therefore the user of the calculator can select a nominal thickness corresponding to a specific pipe product, but they can freely pick diameter and pressure.

With that said, I'll admit that it's strange that diameter is not nominal but thickness is. Typically a pipe sold by a plumbing or vacuum company would specify products using both nominal thicknesses and diameter. But pressure could be user dependent.
But again, with ALL OF THAT said, about why thickness is nominal but diameter is not, I would say if the calculator can accept any values for diameter, thickness, and pressure, because it is using some analytic or numeric formula to calculate the answer, I would say it doesn't make sense to restrict the thickness supplied to being nominal since the user can put in any thickness they like. In this case, perhaps the tool was written by someone who DOES work with plumbing parts and is just used to seeing the word "nominal" paired with the word "thickness" when using or selecting parts.
