Piping isometric is used for used for fabrication and calculate the mto. In piping isometrics all the bill of material shows like number of elbows, flange, piping length, bolts, gaskets. Without isometrics construction of any project us not possible. Isometrics are prepared on auto CAD from piping GA's. If company is using 3d software like pdms, pds, SP3D, Auto plant then isometrics are coming automatically from the software but you to do some changes manually on that isometrics which is produced by software. If any one set the standards properly while producing the isometrics then there is no need of any manually changes on the isometrics and that isometrics can directly send to the fabrication shop for producing the spools.
Check below mention video for producing the isometric on auto cad.
https://www.youtube.com/watch?v=xQbgGsmdhjU