What are pipe fittings and pipe fitting numbers?

hello, i have a lab report that i need to do, and my data sheet says number of fittings. Under that "number of fittings" are numbers for the pipe run or type of fittings. Some examples of pipe runs or type of fittings are 45 degree elbows, straight, 180 degree long and short returns, 90 degree tees and a few more. So my question is, what exactly are the pipe fitting numbers ociated with the pipes? And how do i use that in my lab report? thank you?